For a long time, the go-to if you needed more ports for your laptop or desktop PC was a USB hub. You can still get those, naturally, and they have their value (they tend to be cheap and portable). Thunderbolt docks have exploded in popularity however, mostly because of Thunderbolt 4, which is almost always included alongside USB 4.
As for what people hook up to a Thunderbolt dock, the most popular answer is probably one or more displays. There tend to be multiple USB-C ports that can double as DisplayPort connections, and if there's one thing that's going to stay static on your desk, it's a gigantic LCD or OLED monitor. If that's all you're doing with Thunderbolt, though, you may be missing out, especially if you're venturing into media production. It might even be worth bumping up to Thunderbolt 5 if your computer supports it.
Physical security keys
Always safer at home
There's been a growing shift away from passwords in recent years, mostly in favor of passkeys, which are both more secure and more convenient. If you really want to step up protection, the ultimate move may be a physical security key. That way, even someone with direct access to your computer and the ability to bypass your PIN may not be able to log into a particular app or website. I bought a YubiKey explicitly to protect a bank account when I was moving from Texas back to Canada.
You can keep one ready to go at home without the risks of taking it everywhere, or keeping it permanently inserted in your PC.
The downside to physical keys is that they're small enough to get lost. You're often meant to keep them on a ring next your car fob. But if you have a Thunderbolt dock with enough free slots, you can keep one ready to go at home without the risks of taking it everywhere, or keeping it permanently inserted in your PC.
There are potential vulnerabilities to this approach, namely if you keep your PC docked most of the time, or if you're away from home and suddenly need to access the thing the key is paired to. But those can be worked around.
Audio interfaces
Professional sound in a stack
For full disclosure, I don't use an audio interface at the moment, since that's well beyond my current needs. But I'm familiar enough to say that if you're a musician, podcaster, or streamer, you should almost certainly have one. It's going to allow you to connect mic, guitar, and other instrument inputs, and take advantage of high-quality preamp and DAC technology for recording and output. Audiophiles may want one just for the sake of listening to lossless music on the best possible headphones and speakers.
In a sense, there's no particular need for a Thunderbolt dock, except that audio interfaces tend to be bulky and jammed full of cables. So the further away they are from your keyboard, the better, as long as you can still reach any knobs, buttons, or sliders. Depending on the interface's design, you might even be able to stack it on top of your dock or vice versa.
Note that while some interfaces are built explicitly for Thunderbolt, you'll be fine if one is limited to USB-C. That may actually be preferable, since some other peripherals demand Thunderbolt's bandwidth and power capabilities. Doublecheck what USB-C speed an interface requires, though.
Hot-swapped and semi-permanent storage
You don't need everything all the time
One of the biggest mistakes you can make when building a PC is going overboard on internal storage. While internal SSDs are always faster, you pay a heavy price per gigabyte. And if your computer suddenly fails to boot, you may be in real trouble if there are important files stashed there. External SSDs are cheap, easy to swap, and fast enough for documents and media libraries. Save your internal drives for apps.
In my case, I use a 2TB SSD, loaded with documents and a handful of movies and TV shows. I don't usually need this data when I hit the road, so leaving it connected to my dock simplifies travel without having to spend a fortune on my laptop configuration.
Video professionals depend on Thunderbolt docks not just for libraries, but for ingesting footage, or establishing a scratch disk when working in an app like Adobe Premiere. This is probably the main reason anyone needs Thunderbolt 5, short of connecting an external graphics card (eGPU). Using compatible gear, Thunderbolt 5 is at least twice as fast Thunderbolt 4, potentially three times. That makes a huge difference when the projects you're copying are measured in terabytes instead of gigabytes.
High-end webcams
Stepping up for pro streaming
As a rule, if you own a recent laptop, you probably don't need a separate webcam for your computer. It used to be that built-in cams had terrible resolution and light sensitivity, but now it's de facto for many of them to handle 1080p and dim lighting conditions. No one's going to complain about your image quality in the average Zoom meeting.
The issue is that desktop PCs usually lack their own cams, and regardless, stepping up to a high-end model may be worth it if streaming is integral to your job. Better cameras can offer perks like 4K resolution, higher framerates, HDR support, and in some cases automatic zoom and panning functions. Indeed you're a streamer on a service like Twitch or YouTube, you may look distinctly unprofessional using a built-in camera.
The greatest advantage of connecting to your Thunderbolt dock may actually be the simplest: positioning. You can place a cam just about anywhere on your desk, instead of being stuck with a permanent head-on angle, or going as far as your computer's built-in ports will allow. And no one wants to pull a clip-on cam off their laptop every time they head off to work or the airport.
Printers
Once more, but without feeling
This isn't exactly a new sentiment, but I've come to loathe most home printers. Affordable models can be unreliable, and somehow, they never seem to play well with Windows, which can result in backed-up queues. Worse yet is the inkjet scam. Many of us print so rarely that the ink will dry up, and replacement cartridges can cost so much that it's often cheaper to buy a new printer.
There's a growing community around 3D printing, and moving that hardware off your desk is practically a necessity.
A dock will typically let you move a printer off your desk while still keeping a wired connection, avoiding the hassles associated with Wi-Fi pairing. That's not as ideal as a network-attached printer -- but if you're the only person printing, there's no need to complicate things. Routers aren't always close to your desk, either.
On a more positive note, there's a growing community around 3D printing, and moving that hardware off your desk is practically a necessity. Some of the popular models out there would swallow up half my real estate.