802.11g's big benefit is that it operates in the same unlicensed, 2.4-GHz band in which 802.11b lives. This allows backward compatibility with existing 802.11b gear. Rather than discard old Wi-Fi equipment, you can use old equipment if you opt to move to the 802.11g standard. Unfortunately, many consumer products also use this band, and you can experience interference from a number of sources, including cordless telephones and microwave ovens.
802.11a also operates in the unlicensed 5-GHz band, which is largely uncluttered and offers many more nonoverlapping channels (assuming you use both indoor and outdoor channels) in which to operate than 802.11b and 802.11g.
There is a tradeoff in range when it comes to the frequency band in which a radio broadcasts. 802.11b and 802.11g get a better range because they operate at a lower frequency than 802.11a. Lower frequencies propagate farther than higher frequencies. Longer waveforms also have an easier time penetrating obstacles, such as walls.