Most installers of tile floors do not seal them because grout has to cure first. It's something you should do to keep your grout looking it's best. As a bonus, sealed grout is much easier to keep clean.

Is it really necessary to seal grout?

Usually, grout is made of the cement material, making it porous. This is why it can absorb liquids readily. Hence, grout sealing is necessary for your tile surfaces. In addition, grout sealing is useful because it stops mold and mildew from growing on your grout.

Does sealing grout make it waterproof?

Sealer soaks into the grout and supposedly makes your tile and grout waterproof. People who want to sell you grout sealer will tell you it keeps it easier to clean, and helps prevent mildew from growing.