Rest API for multiple applications?

I recently started learning about REST APIs and I am trying to understand how they work and what they are used for. From what I have read so far and from what I understand, a REST API can be used as a back-end for multiple different applications at the same time, android applications, web applications etc. Is that the main use of a REST API or is it just something it provides but not the main reason to use it. Or am I completely wrong?