- Does it impart flavor onto the food?
- For how long? (which I guess is one way of asking, "How often do I need to do it?"
- Do I still "need" to apply butter etc.. to the pan even though I "seasoned" it? (in this case, maybe it's a question of taste, not need)
Purpose of Seasoning
- protect bare cast iron from rust
- make the pan surface non-stick
How often to season the pan?
- You'll need to season it more when it's new.
- Use it frequently and you'll need to season it less often.
- Don't cook beans or tomatoes in it at first; if you do so later, you may want to re-season it.
Regarding adding butter, you're right: that would be for taste, not necessity. If seasoned properly, a cast iron pan is non-stick.
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 :)
One important rule is to season your skillet (or especially, your wok), if you can see the unseasoned metal of your pan at any time. If you've just made a dish and wiped the pan off and notice there's no coating on part of it, season it while the skillet is still hot.
Regular use of oils creates and enhances the seasoning over time. Some of the lipids create a surface that becomes non-stick, but cooking will "wear" and erode this surface depending on the actual food being cooked. Additionally some of the oils go deeper than just the surface, even if just slightly, an effect I believe is enhanced by higher heat. This does not mean you need to use huge amounts though as that will still depend on the food being cooked.
Flavor: yes, not because of the seasoning, but because of the use - less sticking can create a better sear, retaining the flavor, especially in foods such as meats where it is desired to seal in juices and yet create a surface where the chemical change imparted by heat is enhanced by not being lost due to sticking and integrity of the food is retained.