I've seen this in a few places on desktop, and I have no clue why it's even a feature. I'm not aware of anyone using it anywhere (although to be fair I haven't thought to ask).
As for why it's enabled by default, probably for visibility. The easiest way to get people to use a feature is to make them use it and make them explicitly disable it (if even an option). For AI training, they could theoretically just capture typing data and messages regardless of if the feature is enabled/disabled anyway.