Why increase or decrease rudder when using elevator in turns? In normal 30 degree bank level turns, instructors teach students that if you are to correct your altitude you also need to change your rudder input. 
Why is that?

For example if you are descending in your turn you are to pull the elevator and also lessen your rudder input. And if you are climbing in your turn you are to push the elevator and push more rudder. 

Is this because of you are tightening your turn when pulling elevator and therefore you are starting to skid and therefore you need to lessen your rudder to compensate. And if you are pushing the elevator you are doint the opposite starting to slip and therefore you need more rudder. 

Or is there some other explaination to why you need to do this?

EDIT:

Hi,

no I think you have misunderstood the question. 
They do not teach to use the rudder to compensate for pitch/altitude. 

They teach properly to use the elevator. But they say that when you make a change with elevator to a higher nose you don't need that much of rudder than before. And when you use the elevator to push nose down there is more need of rudder. 

The rudder change is just to compensate the need due to change in configuration. Not to use it to change the altitude.
Instructors that teach that to students are fools.  You should be using rudder only as required to keep the ball centered - period.  If you are applying rudder to influence the pitch attitude, to "help hold the nose up", this is very very bad, and leads to stall/spin accidents.  

Rudder is mostly being used to counteract adverse yaw from the aileron inputs, and to some degree to correct for yawing motions caused by power changes.  But in any case it is strictly used to keep the tail lined up behind the nose in the airstream.  Anything else and you are skidding or slipping in the turn. 

If you are in a 30 deg banked turn, and are descending, the total lift from the wings is insufficient; you need more elevator input, NOT top rudder, which creates a slipping turn, which will make the airplane flick over the top into a spin if you stall while doing it.  

Don't overthink it. Ailerons to keep the bank angle, elevator/power to control the pitch and altitude/speed, rudder to center the ball; just do whatever it takes.