The purpose of sealing the grout is to protect the surface from soil and stains. The grout lines are always below the surface of the tile and behave as "sinks" or "gulleys" where soil and water can collect. Each mopping, while improving the appearance of the tile will further soil the grout lines. As the mopping progresses, the water becomes more and more soiled.
The soiled water is deposited in the grout lines where the absorbent, rough surface soaks it up and holds it in (anyone who has ever had light coloured grout knows it can turn jet black very quickly). The purpose of grout sealer is to prevent the grout from absorbing and holding the soil. It will still be deposited on the grout but it will be easier to remove by general cleaning and much less likely to cause a permanent stain or discolouration. Clear sealing slows this process down and gives you time to clear any spillages up.
Clear sealing can also be used on porous surfaces like. Limestone, sandstone, concrete, terracotta, quarry tiles and slate.
