Wow I almost feel singled out by this. But I presume that this is supposed to be in public. Which by every account is a dick move.
However I do this in private, I.e. in my car or at home. And the reason I do it is because I can’t hear a call without using the speaker phone. Hearing issues aren’t fun.
However the easier solution is headphones, but calls over Bluetooth can make things worse, from battery life to sound quality.
Wired headphones are the preferable solution to this but we all know what happened to those.
TL:DR I do this because of my hearing issues. In public this is a dick move, but headphones aren’t the solution they use to be.