They are for moving the currently selected tile without requiring the mouse.
I think we are not talking about the same thing. Under the 'Edit tiles' tab, there is a group for specifying the tile position and another one for the tileset. I was talking about the tileset group.
Yesterday i didn't see the effect of modifying the position of the tileset, but this morning i've found that you have to select another tile in the list to see the effect. The x,y position for the tileset is kind of an offset within the tileset i think.