Do you need to put a sealer on sanded grout?

You don't necessarily "NEED" to seal it, but it is a wise choice! Grout is a cementitious material just like concrete and both absorb water unless they are waterproofed. Absorptive concretes develop structural issues and require repairs with chlorides and other contaminates which enter and cause reinforcing steel to corrode.

Grouts merely absorb stains and discoloration becomes an eyesore. Mop water that isn't changed often enough when cleaning will be absorbed and turn grout joints dark, and spills are wicked into a grout within seconds.

I have beige tile and grout in a kitchen/family room area that is six years old and looks brand new. Sealed with a product called StableCrete to waterproof the day after it was installed. Common response to visitors walking in, "What a beautiful tile job, is it new?"