I'm not aware of a difference on the Retool side of things. Once url parameters are supported they should also be supported for POST requests.
The platform you're making the request from might not let you make a POST request (see this thread, for instance) or might have certain restrictions on the configuration for POST requests, so it could be useful in certain cases to use GET instead. It can also help indicate the purpose of the call in the code where the call is being made.