Why is it important to use voices of stars?

I don't understand the reason so many animated films are casting stars for the characters voices except as a gimmick to attract more adult audience. Their star quality doesn't come across as the voices of cartoon characters. Jason Alexander as the father swan doesn't even sound like Jason Alexander. Voices of singing stars for the musical numbers would make sense to me. In the past, some voices were done by actors who specialized in voices, and one actor, such as Mel Blanc (http://www.imdb.com/name/nm0000305/), often did the voices of several characters.