Tile does not have to be sealed but can. Think of it as "waxing" your car. The sealer can't be absorbed into the tile because of how impervious it is. There is such a minor amount allowed to be absorbed that it's usually not enough to stain the surface. So why should I seal the tile if it's stain proof? The answer is more related to routine cleaning. You should include sealing as part of tile care that helps with routine cleaning. Only high end sealers are recommended as they generally will last much longer against normal use. Impregnator types are the professionals choice for grout and natural stone but can be used on ceramic and porcelain tile with great results!
Stone care is more of a requirement when it comes to sealer. Unlike ceramic and porcelain tile, natural stone can and will absorb liquids. Even a sealed stone will absorb stain causing oils and juices. A misconception is that a properly sealed stone finish is stain proof. A more accurate comment would be that a properly sealed stone finish allows you enough time to wipe it off before causing damage under the surface. If you let any acidic liquid like orange juice sit on your granite for long enough, it can "etch" the surface which creates a dull look. It basically eats the highly polished surface which cannot be repaired unless you hire a professional stone restore company which costs a lot of money. Do yourself a favor and wipe those spills up and use a high quality impregnating sealer to protect your investment!
Grout should be in the same camp as stone. An unprotected grout area is like a sponge. All those tiny little spores or pockets can trap dirt and grime making it very difficult to clean. The idea of sealing grout is to fill up all those little pockets and spores making it less likely to stick and easier to clean. The same idea in wet areas like showers. Another common grout issue is to keep the mold and mildew slime from setting below the grout surface. It makes cleaning much easier. I like to use a high quality impregnating sealer that will last at least 2 years before re-applying. Also keep in mind that a properly sealed grout job may appear to get stained on the surface and is usually just the dirt and grime from everyday use. Read about how to clean grout and you will realize that a protected grout surface usually looks like new once it's cleaned!
