There are several good reasons to follow up with a professional grade clear grout sealer after your tile has been installed or you buy a new home with ceramic tile.
One of the reasons your tile and grout gets dirty so quickly is because grout is porous. That means when you spill liquids or track dirt over your floors, your grout soaks them up. Sealing your grout with a clear sealer will seal your grout pores, protecting your grout from staining and keeping it looking (and staying) cleaner longer. A clear grout sealer will also make your regular floor cleaning easier and more productive.
Applying a clear grout sealer after you purchase a new home with ceramic tile or after your tile has been installed (if the grout lines have not already been treated with a clear seal) is one of the best ways to assure your tile and grout will remain looking great for the long term.