Of corse it is a poor substitute for an actual penis. But it is a lovely supplement. You could bury into that ass full hog and spend yourself in 15 minutes. But if you have a voracious bottom sometimes you need something that stays hard longer to satisfy him. Or, say you are playing with a beginner. Impaling them can hurt like a bitch. Opening them up with a nice toy can ease them into ass sex.
But, I think the question you are asking is if we prefer a dildo or a vibrating dildo, as vibrators are those egg shaped things or multi-pronged rabbits. But I don't have a preference. I like the right tool for the job.