Because it cooks things quickly, eg something like cauliflower soup takes only 5 minutes at pressure. But also, once you've put it all in you can walk off and leave it and the IP will keep it warm for you for hours.
So, i can chuck all the soup stuff in and leave the house to go shopping or whatever. When I come back it just needs blitzing and it's ready to serve. No need to watch it or stir etc.
You can fry stuff in it first before switching it to pressure too, unlike normal slow cookers.
Something like a lamb curry that I'd usually simmer for 2.5 hours will be perfect with about 25 minutes at pressure. Bolognese takes about 15 minutes. Chicken curry/stew takes about 12 minutes.
So, kind of like the convenience of a slow cooker, but the taste and texture is so much better than slow cooked. I find slow cookers reduce everything to mush.
I just love that I can get a decent home cooked meal on the table after work without too much of a wait for us.
i use mine about 3 times a week and I promise I'm not on commission!