Also gives you the ability to change the color of the grout.  For example,  if the original grout was white but now they painted and want the grout to be antique white. 
It also makes the grout a uniform color.  Sometimes even brand new grout can be different shades from different lots.
Some of them contain an antimicrobial.
It provides more protection than an impregnating sealer because it is a topical.