Phone number, I guess. Sending SMS messages en masse is expensive or heavily regulated in much of the world and it doesn't particularly suit frequent "spamming" so people of a younger generation may be more willing to hand it over(?)
Yeah, I have a number that I "hand over" and another number I don't. The public number is inundated with spam SMS to the point that I don't even bother checking them anymore. And this is with pretty conservative behavior: even though I have a dedicated number for this purpose I still avoid entering it unless I absolutely have to.
