It's a feeler (IMO way more descriptive than "doorknob") that lets you bail if you don't want to talk. Without it you may feel obligated to listen or have to be

It's a feeler (IMO way more descriptive than "doorknob") that lets you bail if you don't want to talk. Without it you may feel obligated to listen or have to be slightly rude. With it you have a range of options like "swamped on this project, catch up with you later" or such depending on circumstances to avoid straining a relationship. In async communication it's unnecessary though, people might do it by inertia.