Because it lets you test the whole stack and replace a mocks setup with normal code (endpoint option and simple plug). It lets you test how your payloads are actually encoded for instance.
In general I do two layers of mocking. I use Req.Test to test actual client inmplementation, and a mock client to return mocked responses directly (simpler and faster, as you said) for code using the client.
But for an app tha is mostly an ETL software with heavy HTTP fetching, authentication layers, configurable pagination, that kind of things, all related test start a tiny HTTP server sand I have no regrets. It’s actually fast enough, test can be async, and I trust the whole stack to work well.
. Mox actually uses some OTP-code patching in runtime in order to make coverage work, while Repatch just does it the correct way.
