what if we delete more key-frames in the x-axis to get more smooth curve? if we can't, why?
You can delete keys if you need/want, but when you are doing more complicated animations, it is best to have all of your keyframes nicely lined up on the same frames. That way if you need to move keys around they are going to behave in a sensible manner.
This will make more sense in the later animations - but for now, do whatever works for you. Just pay attention to what you did, so you know where the keyframes are and where you are just using the handles (if that makes sense)