What is the purpose of seasoning a skillet?
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.