It's important to remember that aside from "convention" (and likely having a few helper functions defined in whatever server library you're using), there's nothing special about the method/verb in the HTTP protocol. You could literally define FOO and BAR methods in your API and use those if you found them useful.

That said, PUT and PATCH can be quite useful if you're dealing with CRUD operations. If you want or need your API to be idempotent, use POST for creation and updating. If being idempotent isn't a requirement - or even a negative - then adhere to the convention: POST for creation and PUT/PATCH for update.

PATCH is really nice to support when your objects are even moderately large or you want to reduce bandwidth costs. For example, let's say you have a user object in a NoSQL DB and each user is a JSON object with 100 keys. If you use POST or PUT to update, you're basically requiring that the user of your API...

* Perform a GET to actually fetch the entire user object * POST/PUT the entire object back with the changes

In addition, you're likely going to want the API to track the version of the object, otherwise you run into issues with multiple clients trying to update at the same time. This can be very problematic when updating the entire object. So you're basically wasting cycles on the server fetching data, wasting bandwidth for data that isn't going to be used at all, and possibly writing extra code to handle conflicts.

With PATCH, you don't need to GET anything. You can update fields pretty easily w/o even knowing what's already there:

PATCH /user/12345 { "age": 10 }

Very little bandwidth is used. If you still need versioning along with that (yes, you do if this is more than a pet project) to resolve conflicts, then you can use a HEAD request for the user and return the version of the object in an HTTP header. Multiple patches from different versions can all be successfully applied on top of each other until a conflict arises. But, then you're getting into vector clocks or other solutions that are outside the scope of the question.

Circling back, yes, PUT and PATCH can be very useful. ;)

HTH