Hey maryjane22 - I would say for several reasons....
#1. It is the quickest way to see how a certain strain grows, flowers, and finishes up. This might be helpful if you are trying to find a mother for vegging in the future.
#2. More plants can be grown in a space because footprint is smaller. Still equals quicker bud than vegging one plant.
#3. Trying a new technique because I am bored with vegging mothers, and cloning.
#4. More opportunity to find a decent phenotype, since you are growing each plant from a new seed instead of sister plants that are exactly the same.
#5. Saving electricity. No need to have any light burning for 24 hrs a day. Only 12/12 light cycle. My light bill dropped this month because I'm no longer burning a MH light all day long. Same could still be said about CFL's or HPS if you are using them to veg.
#6. Eliminates need for separate veging cabinet and hassles for ventilation, etc.....
There are prolly more advantages and several disadvantages not discussed here that people could add......
PB