How come you take a pee test every time you go see your obgyn

For urinary infections. If you use someone else's urine, that person may have an infection and you will be given medicine that should go to the other person.

Do they use the same test to c if you are pregnant

I don't think so. The doctor wouldn't check for that unless you ask him to. Are you pregnant?

Urinary infections can be very bad, since your reproductive organs are right next to your urinary system organs.

When you give urine at the doctors office pregnancy isn't usually something they test for, unless you request a pregnancy test. At least not where I live.

Urine is tested for infections, or abnormalities. It's a standard test when you go for a physical.