Sealing is done to strengthen tile or grout's defenses against dirt, spills, and erosion of grout, but not all tile needs to be sealed.
If your tile is ceramic or porcelain, it probably won't need to be sealed. There are some exceptions to this, so check with your manufacturer to be sure. If your tile is stone (slate, marble, granite, travertine, etc.) then it will need to be sealed. Stone is naturally porous and will absorb spills and stain fairly easily.
You can sometimes tell if your tile or grout have been sealed by spreading a few drops of water on them. If they darken or change color, they are probably not sealed. If they stay the same, they may have already been sealed.
It's always a good idea to follow your manufacturer's instructions on a schedule for sealing grout and tile.