this will retard that mold, it will keep it cleaner longer and it will not allow water to seep into the back side.
Now, sealant is best applied with a small foam brush. You only seal about three days after the grout has cured out. Then the sealant is applied in two or three steps. You want to achieve a beading of water on the grout.
Existing grout cannot be properly sealed, only new grout. May be some disagreement on this statement, but I adhere to this.