Good and thorough
You may think tile floor are easy to clean. The use mops, swifters, and the combination of over the counter cleaners just does not get the job done.
Mold and Mildew, debris get stuck into the pores of the grout and tile over the years. Soil build up normally occurs gradually so it may not be obvious at first, while the tile and grout are becoming a darker shade of the original tile color.