Playing single notes on consecutive strings may be difficult when relying only on the alternate picking technique. The permanent direction shifts of the pick, while necessary in most cases, become encumbering in situations such as this:
They take a considerable amount of time, making it almost impossible to perform these kind of licks in a fast and clean way.
Sweep picking is a guitar technique that was largely developed in the 1940's, where the guitarist uses a single constant downwards or upwards pick motion to play a series of notes on consecutive strings. This allows a guitar player to perform fast arpeggios or arpeggio-infused licks and melodies, natural to instruments such as the piano, but unpractical for the guitar when relying on traditional picking only.
Nowadays sweep picking is a standard item in every guitarist's technique repertoire, with a number of common fretting shapes being basic tools for improvisation. Sweep picking is usually combined with other techniques, both to create interest and to extend the possibilities it offers.