why when you study math or physics you are obliged to know how this or that equation is derived? every time you learn a new equation they tell you to prove how this formula was derived, but why is this made ?
i mean the derivation of an special formula is applied once and can not be used to prove another formulae
so why in the process of learning you must know how to derive each math formula you learn ?

I believe there are many different kinds of good answers to your question, but I'll just focus on one:
If you're lucky, you may indeed go through life and successfully apply the formula without being able to derive it.
However, you may find yourself in a situation that is slightly different from the kind of situation that the formula was intended for.  In that case, if you knew how the formula was derived in the old kind of situation, you may be able to change that derivation to derive a new formula that is more appropriate for that new kind of situation. 
Indeed, if you know the derivation, then you are immediately more likely to know the assumptions being made for that formula, and thus be able to gauge whether it is appropriate for you to use the formula in some kind of situation or for some problem in the first place, by seeing if those assumptions indeed hold true for the situation or problem you are faced with.
