I like that repl.it lets you quickly test code on almost any device with a web browser, without requiring the user to download or install an IDE or compiler. The platform is simple and easy to use, making code testing while I'm not on my main computer a breeze.
I don't like that free users are not allowed to use private repls. I believe that free users should at least have two free private repls that they can use for testing that they might not want everyone to be able to access.
Even though its probably best to use a paid account, there's still a lot you can do with just a free account! Before you take out your wallet, make sure to give the free platform a whirl, odds are that you'll probably just need one repl for quick code testing and in that case the free plan is perfect.
This is definitely a good fit for freelancers that have to show a client running code or for organizations that want to empower their employees with a valuable tool for on-the-go use or for demonstrating functioning code (or even debugging that code!).
Instead of installing an entire development kit on a computer I might only use once, I can simply use repl.it with a web browser instead, saving valuable time and ensuring that I can access a testing sandbox from anywhere, for most major languages.