I assume you don't mean keyboard text predictions, which would be a different thing, but the platforms.
It's a new convenience feature. Something they as a platform can shine with, retain users, and set themselves apart from other platforms.
Having training data is not the primary potential gain. It's user investment, retention, and interaction. Users choosing the generated text is valid training data. Whether they chose similar words, or what was suggested, is still input on user choice.
It does lead to a convergence to a centralized standard speak. With a self-strengthening feedback loop.