Using a WebSocket server is a great way to speed up your applications. APIs inherently come with their own HTTP overhead, which means every time you call an API, you have to wait a little bit for the HTTP response.
This is mostly fine, but if you have an application with a lot of time-sensitive and frequent server requests, it can become a problem. A good example of this is a chat app, where you need to see what the other person is saying immediately. APIs can still work in this scenario, but it's not the best solution for the job.