Pressure cooking is very common in some countries, but I'm only familiar with the simple mechanical pressure cookers (that work on gas or induction stoves). Can electric pressure cookers do things that a regular one can't?
Being able to dump ingredients in and set the pressure for 20 minutes, and then go out for 2 hours and come back and eat warm food that hasn't gone bad is really, really nice.
Apart from that - as others pointed out, they're multifunctional. I make yogurt in mine. Also acts as a slow cooker but in my experience not very well.
In general I would say that the temperature control in an electric pressure cooker is going to be more precise, so you see functions like yogurt making in them, too.