For most things 16 hours of florescent light a day is enough to give your seedlings a good start. You can also give your seedlings 24 hours of light a day for the first few weeks after they come up. This is very effective with things like geraniums started from seed. After the first few weeks cut the light down to 16 hours. If you give them 24 hours of light for too long they will try to go to bud as soon as you cut the light down to 16 hours or move them into natural light.
Many seeds require light to germinate.
Check the variety before you plant. 24 hours of artificial light will help speed the germination process. This is when the clear plastic covers are great because they keep the soil surface moist while the seeds get going. Just remember to keep them out of the sun. The distance the lights are kept from the plants is critical.