Now this is MY opinion again. I have time and again told MY customers that I question the real benefit of applying sealer to ceramic tile grout. No doubt there is some benefit, but it is similar to applying clear nail polish to nails. How much it protects it is questionable. And I have always wondered, too, how often it has to be REDONE!
Usually I have seen customers who have always apologized for their sealer. "Must have put it on wrong", or "put it on too late, I guess"... whatever. The only time I have used it in my home (applied by a professional) it happens to be the dirtiest and/or darkest my grout has ever looked! I was/am very disappointed by the result.
This is different than applying it as a decorative finish to porous products like saltillo, or slate, or travertine inside a home. But I am left with wondering the real benefit of applying any to grout where the product is "tile".