It ensures that they do it the same way

What do you think standards are?

Documenting the commonalities between implementations in order to gain interopability, is precisely the point of a standard.

> Conversely, what is the point of a standard that just documents what 2 out of 3 multi-billion companies want to do anyway?

Because it provides a fixed target that can be targeted with confidence by both developers and also the makers of any alternative browser (of which about 4 are still listed in global browser market share statistics).

Those who complain that right now the browser market is dominated by a couple of players seem to be entirely oblivious to history.