Bathroom tiles need maintenance if you want them to last a significant amount of time. And though they are designed to hold off water, they will need drying form time to time otherwise they can attract a significant amount of dirt.
Nothing makes tiling look clean then rerouting, and if your grout has blackened between your tiles, digging it out and re-grouting can give it a much better outlook. For darkened tiles and grout which is not severe, white spirit vinegar can help clean the marks and by applying it with an old toothbrush you will see a significant difference.
Of course if your tiles are damaged, there would be no need, and tiled will either need to be replaced, or you may find your bathroom needs completely retiling. Either way, it’s important to know that bathroom tile maintenance is important, and tiles should never be left alone without any care and attention.