It's like a library that you can call regardless of language ecosystem.
I joined a company that had a large, old, and mature selection of services written in PHP. If I'd tried to rewrite that mature code in a different language I probably would have wasted a lot of time for little benefit. If I'd had to write new code in PHP just to access old code as libraries that would have been a problem too. But functionality was exposed over HTTP APIs that could be used from any language, any runtime.
Well you don't want to call a processor intensive task with a library. A separate service for these types of tasks is a better architecture.
Although I'm not sure whether or not a task queue architecture with separate server workers executing code passed over from a central monolithic app server is still considered a "microservice" architecture.
