They don't need announcements. Literally. It's just an added "benefit" reminding people to brace themselves or whatever, incase injury occurs.
Basically they do it in the name of H&S, of which does not need to exist because it's obvious. To further prove this: Spinball, when changing to sonic, replaced their "heads back, hold tight!" announcement to a plain and simple "Let's get spinning!".
RMT at towers does the announcement purely for interaction.