The theory is, I guess, that creating an MCP API is a lot easier than creating a regular API. A regular API is a very costly thing to develop and it has on-going costs too because it's so hard to change. You have to think about data structures, method names, how to expose errors, you have to document it, make a website to teach devs how to use it, probably make some SDKs if you want to do a good job, there's authentication involved probably, and then worst of all: if you need to change the direction of your product you can't because it'd break all the connected apps.

An MCP API dodges all of that. You still need some data structures but beyond that you don't think too hard, just write some docs - no fancy HTML or SDKs needed. MCP is a desktop-first API so auth mostly stops being an issue. Most importantly, if you need to change anything you can, because the LLM will just figure it out, so you're way less product constrained.