That was the other thing. It was impossible to get my grandparents to do this. I think I eventually got them to let the answering machine answer and not return calls for messages from anyone they didn't recognize.
Because I'd like to think I'm 'someone competent', and on my own phone I only accept (non-personal) calls that I'm expecting, or can see a reason for.
Over time it should allow Google to flag numbers as potentially being a scam, marketing, etc based on other users flagging a caller. Saves Googling for that information which is what I currently do with an unknown number. The only risk is spoofing/false positives putting your number on some blacklist.