If I know the quadratic formula, why must I also know how to complete the square?
At least, it may help you to understand how the quadratic formula was derived.
I have to say that there was a time in high school that I had the same thought. I later came to understand that mathematics is not simply remembering and applying formulae but, rather, questioning how and why these formulae came to be - and so much more besides.
Have faith - It's worth it!!