The purpose of grout is, simply, to fill the joint between the tiles and becomes a permanent, integral component of the finished installation. Grout comes in many colors however the color is aesthetic only and has nothing to do with the function of grout.
Most tile installations use cementitious grouts. The grout should be sealed with a penetrating sealer, grout sealer, which does not contain silicone. Grout sealer will not change the color of the grout but it may darken the intensity of the color. Test first. The sealer prevents the penetration of moisture, simplifes maintenance, and help prevent staining or discoloration. Only the grout needs to be sealed, not glazed floor or wall tiles.
You should reapply the sealer on an annual basis depending on the sealer quality, traffic patterns, and maintenance routine. Refer to the manufacturer warranty, technical & product information for specific details on product installation, useful life, and product applications (including any warnings) before use.
Epoxy grouts are harder to install but are chemically cured and acid resistant and, as a result, do not require a sealer.
Neither sealing the grout nor using a 100% Epoxy Grout will guarantee against surface build-up or discoloration of the grout. Grout needs to be cleaned on a periodic basis to remove any surface build-up. Routine grout cleaning can be done with a daily concentrated household or commercial cleaner depending on the application. When heavy duty grout cleaning is required, you will need to use a professional strength Tile & Grout Cleaner that is capable of removing grease, soap scum, body oil, mildew stains, algae, and synthetic or acrylic waxes from the grout joints. However, such a product should contain non-polluting chemicals and low VOC levels. After cleaning reapply the grout sealer.