Well, sadly, smooth animations sometimes need extra keyframes, but you might want to learn how the frequency must be, so that you don't add too many.
When it comes to rotation, values over 90 degrees will require an extra keyframe. Anything under only needs start/end.
When it comes to movement, you always want one keyframe between the start frame and the destination frame. If those are very far away from eachother, like in a decay animation, you might want to throw in three or four.