I think one of the best reasons is because for users that have some experience with the technology want to be able to quickly find the documentation they're interested in. Can this API do X, for instance. And if it can what is required, what is optional and what are the ranges for values.
I don't want to wade through lots of text and other clutter. I'd much rather have a tutorial or training be a different thing I can use when new to the technology.
Those are valid points, but that just means the API documentation and the concept documentation should largely be separate documents, not that the documentation doesn't belong. In the best cases a link over to relevant concept docs from API and vice-versa is great, and occasionally enlightens even hardened devs ("wait, there's an API to do this directly?", "Wait, this concept was added and streamlined in the 3 years since I first used this product?", etc)
I don't want to wade through lots of text and other clutter. I'd much rather have a tutorial or training be a different thing I can use when new to the technology.