Umm, while that is indeed alternate picking, it is not really the main use I don't think. Alternate picking is simply changing direction of picking for each note played, even when skipping strings. Alternate picking is used for speed and accuracy, as well as tone, as it creates even tone throughout single note solos. Sweep picking is the different type of picking, in which you pick in the same direction between strings, so that you can get fluid arpeggios.