Hand percussion is associated with places and cultures all over the world. There are African hand drums and wooden guiros from Cuba; bells, cabasas, maracas, wood blocks with mallets, shakers... and the list goes on. Each percussive instrument makes it's own sound and adds a note of complexity to the music. It's easy to overdo it with these funky instruments: too much percussion and a wooden agogo may not sound right with a cowbell, for example. Sometimes you'll need a hand instrument to keep the beat for the rest of the musicians as they add rhythmic complexities of their own.
Percussion in the Song
Percussion instruments break down into four distinct playing categories: those meant for plucking, stricking, shaking, or scraping. They aren't meant for harmonization, but rather for adding depth to the rhythmic structure of a piece. Some do produce a pitch though: a glockenspiel or xylophone are counted among the pitched percussive instruments, for instance. Hand percussion instruments are more for rhythm keeping and are great for singers who want to maintain the beat or even lead the rhythm as they sing.