That depends on what you are cooking I think. Most things I am normally inclined to cook require me to attend to them while they are cooking. Slow-cookers eliminate that and do other things instead, like go to work.

Even if you are cooking something relatively hands off in your oven.. how often do you leave your home while your oven is on? You are locked down to that location for the duration of the cooking time.

Yes it does eliminate prep time. You put a bunch of stuff into a slow cooker, add a can of soup like cream of mushroom for flavor, and it comes out amazing when you get home. Just don't end up eating too much meat this way. Use a lot of vegetables.

You don't have to stand over a stove watching stuff fry. You only have to clean one dish. You make enough for leftovers too.