Boiling serves a few purposes when making beer. Preeminent is the isomerization (changing of the structure of molecules, in this case making them water-soluble) of the alpha and beta bittering acids from hops -- the longer hops are boiled (up to 75 minutes or so), the more isomerization occurs, and the more strongly hop bitterness will be perceived in the final beer. A typical hopping schedule will include an addition of hops [hop addition] at the start of a 60-minute boil for bitterness only, another addition at the half-hour mark for hop flavoring, and another at the 50 or 55-minute mark for hop aroma.