What is the purpose of seasoning a skillet? I've seen a couple questions on how to season a skillet, and this may be a dumb question, but why do you season a skillet? 


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.