I've talked to many people who have purchased Raspberry Pi's without knowing what to do with them. Besides just using one as a normal computer, people like to use them for different projects. Here are some ideas that I've came across since owning a few myself.
Pi-hole is a network wide ad-blocker running from your pi. This was the first thing I did with mine and I definitely don't regret it. This will block ads for all devices connected to your network, whether it be phones, tablets, or laptops. And it's super easy to install!
Raspberry Pi's are also great for programs that you want to run for extended periods of time without getting interrupted. For example, you can build a Twitter streaming application which monitors hashtags on twitter and writes the data into a database, like I did on this post Run that program on a Raspberry Pi and let it go for a month or two, and you'll come back to a plethora of data to play with!
This is one that I plan to do in the near future. You can buy a camera module for the Raspberry Pi and use it for a security cam. You could set up the pi to be watching your front door and build a program to monitor and detect any new objects that come into view. If it finds one, you could have it send you a text or email, letting you know someone was at your door. Luckily, there's already a great library in Python for doing this called PiCamera.
A Raspberry Pi is the perfect learning device, which is part of the reason why it became so popular. Why not dip your toes into the world of creating websites by starting with a Raspberry Pi? Don't worry if you don't have any experience; it's easier than you think! I'd recommend checking out Flask to start off. You can also check out this post I made on Flask, which serves as a good introduction to making websites.
This can be tied in with data collection, but Raspberry Pi's have tons of great sensors you can connect to. You can monitor temperature, humidity, etc. You could get multiple and even work on creating your own website or dashboard which display different information like the temperature outside/inside, humidity, moisture, etc. There are endless possibilities here.
Build a display of sports scores/stock market updates
You know the ticker at the bottom of ESPN? Why not build the same thing but using a display for your Pi! You can buy matrix panels that can be used for the display. Then use different API's to access scores of your favorite sports teams, stock performance, etc and display it on the screen.
Magic Mirrors have been extremely popular over the last few years since the Raspberry Pi came out. It's a great project which gives you experience in coding (if you want to) as well as some hardware things with building the mirror. And it looks awesome when it's all done!
This is probably the most popular thing that people do with their Pi's. If you want to feel some nostalgia and play a bunch of old NES and Super Nintendo games, you use your Raspberry Pi as an emulator for these systems using Retro Pie.
Raspberry Pi's have been great for teaching people more about computers. Try learning a bunch of Unix commands so you don't need to use the GUI anymore (this would be a good start), learn how to SSH from your other computers to the Pi, learn how to write a simple Python program or bash script, learn anything. That's what the Pi's are great for.
That's all I have for now, but the possibilities really are endless when it comes to Raspberry Pi's.