Surely if you answer the call or the door, you'll then find out what it is people want?
I've had:
'do you know where xyz is...' - police looking for a vulnerable missing person with a link to my address that was outdated. I didn't but that ruled out one possibility and aided finding them more than had I hidden!
'We've come to update you on a crime you reported a few weeks ago' - police again, no idea why they knocked instead of emailing or calling but it was nice to know that the man who made threats to me over the internet had been spoken to and was not capable of doing what he'd said he would and was in fact very sorry for his behaviour.
'Your Sky dish is about to drop off' - neighbour.
Loads of parcels that didn't have tracking that would have ended up at the hard to get to depot, gifts I didn't know were being sent etc.
I'd also miss phone calls from the people who want to work with me, or the people who are trying to help me with various things.
Sometimes I can't get to the door, too many dogs in the way, not enough time to get out the room, herd dogs back in, shut door, etc (takes a LOT longer in a wheelchair!) but generally, I answer the door and if I don't want whoever/whatever, I tell them to go away, level of politeness closely linked to level of annoying request!