Subwoofers are commonly associated with surround sound setups, but so-called '2.1' systems are becoming increasingly commonplace. The addition of a subwoofer to your setup will ensure good reproduction of very low-end frequencies, leaving your main monitors to cope with the rest of the spectrum. As with main monitors, care should be taken with the positioning of the sub, and advice on this (and possibly test audio files) should be provided with the unit. Whether or not you need a subwoofer will largely depend on the type of music you are involved with - a standard rock outfit will not trouble a subwoofer too much, whereas it is very difficult to tell what is going on at the extreme bottom end of most types of dance music without one. If you're considering adding a subwoofer, bear in mind how it will be connected, in particular whether it has its own crossover and whether it is easy to bypass - some subs require additional equipment in order to provide these facilities.
"ESI SW10K" Active Subwoofer