Language is about communication.
This line of thought stems mostly from two observations: one, that the vast majority of language you use is your internal monologue; and two, that this internal monologue would have been extremely helpful for the hominids that would have first developed it even while the rest of the population had not developed language (in contrast, if language is about communication, then it's only useful to a hominid if the whole population speaks language, but then it's hard for it to spread initially).
Yes, and we could argue that those are not thoughts, while there still being a distinction between thought language (which could very well be subconscious) and inner monologue/spoken language.
Also our natural languages restrict information bandwidth to a few bytes per second. Imagine doing sports like tennis, chess or soccer at this speed...