Does it impart flavor onto the food?
Not necessarily. The seasoning helps to develop a good sear on the food, and this adds to the flavor. If the seasoning is very old or if the pan is cleaned infrequently, then some flavor may come from the cook top. Old diner often attribute the flavor of their burgers to the grill that has been in place for the last 60 years and never cleaned, but I'm skeptical that this really impacts the flavor.
For how long? This depends on how often the skillet is used, and what you use it for. Baking cornbread seems to strip the seasoning off my skillet as does cooking tomato dishes without much oil in them. I scrub under water with a plastic sponge after each use, and if it looks dry, I apply a bit of oil to maintain. Properly re-seasoning takes too long, but I probably do it 1-2 times per year, and use my skillet frequently.
Do I still "need" to apply butter etc.. to the pan even though I "seasoned" it?
Probably. Foods like onions won't brown nearly as well without some butter/oil in the pan to transfer the heat. You may or may not need to apply oil to keep food from sticking, but it is never a bad idea :)