A.) Attract new readers who were scared to jump on titles bogged down by years of continuity B.) Attract younger readers who wouldn't read comics otherwise C.) Reinvent the Marvel universe as to make it more 'realistic' and grounded in reality--I.E. less flashy costumes and fantastic origins
There was this whole "mission statement" concerning the Ultimate line at the back of the USM HC, and one of the biggest perogatives was to promote it to younger readers in places outside of comic stores. USM #1 was reprinted and sold with a lot of Spider-Man related toys. I used to see quite a few Ultimate-related kid's merchandise at stores. There were also kid digest versions in some bookstores. (With Ultimate X-Men, the sex and the gore were cut out.)
One of the things that USM *was* criticized in this regard was that a lot of Bendis's early arcs were lengthy in set up and would have issues where Spider-Man wasn't in his costume at all. But it still attracted young readers--a five year old wrote in to one to the letters once. USM has gotten a bit more risque over the years, but it's the *only* Ultimate comic that I'd still recommend to a young reader.