If you start tiling from the bath using a full tile, and wing it, you could be using more tiles than you need, and spending ages cutting small bits of tile to fit in small gaps you’ve caused, and making a mess of it. If you start full tile next to a wall when tiling a floor, you can cause the same kind of problem when you get to the other side of the floor.
So setting out your floor or walls before you even mix any adhesive, to the point of knowing the size of every tile you need to cut, is the aim of setting out your room.
With most tile types that are not large-format you’ll need to create a gauge staff or gauge rod (like a brickies ‘yard stick’. Large-format tiles will take up so much space on your average wall or floor that simply having an accurate measurement of the tile width and length/height usually does just fine.
…. working on this page.