Why you need grout.
Even if all tiles look like they are the same size, they are not unless they are “rectified”. Tiles will differ from tile to tile, if only a tiny amount. Butting the tile together will result in the tile not lining up properly, giving you an irregular overall look. Also, not grouting tile leaves the small spaces in-between open to the possibility of all types of unruly things filling them in. Then you have to clean the spaces out risking the possibility of damaging one of the tiles.
Finally, there will always be some movement in your home and on your tile. The expansion and contraction of wood or concrete will do the same thing. Although you can minimize movement by different underlayment materials, it will still move. This will cause the tile to rub together which could eventually chip or crack. If you are concerned about your grout cracking, caulking between two products will eliminate the cracking grout. An example of this is between your tile backsplash and the counter.