If you have worked on the multithreaded code, then you might wonder, why not just use threads? There are several reasons why threads are not popular in Python.

Firstly, threads need to be synchronized while accessing shared resources, or we will have race conditions. There are several types of synchronization primitives like locks but essentially, they involve waiting, which degrades performance and can cause deadlocks or starvation.

coroutine has well-defined places where execution is handed over. As a result, you can make changes to a shared state as long as you leave it in a known state. For instance, you can retrieve a field from a database, perform calculations, and overwrite the field without worrying that another coroutine might have interrupted you in between.

Secondly, coroutines are lightweight. Each coroutine needs significantly less memory than a thread. If you can run a maximum of hundreds of threads, you might be able to run tens of thousands of coroutines, given the same memory. Thread switching also takes some time (a few milliseconds). This means you might be able to run more tasks or serve more concurrent users.

The downsides of coroutines is that you cannot mix blocking and non-blocking code. So once you enter the event loop, the rest of the code must be written in an asynchronous style, even the libraries you use. This might make using some older libraries with synchronous code slightly difficult.