It is a strange concept without actually seeing how it works.
When you look at a room, especially one with lots of different areas like windows, showers, baths etc you need to know at what height everything is. So don't know for certain how the tiles will work in the those areas, so you need to gauge the room. You may have full tiles off the floor, but a rubbish cut under the window or maybe the ceilings out and you get a rubbish cut there. You need to know before you start tiling otherwise you could drop a massive clanger.
Draw a line around the room, it doesn't matter what height, it just has to be level. Now you can measure from this line around the room and check where the lowest point of the floor is and if the ceiling is out of level. These are important to the setting out.
So if you want for example to have a full
tile off the floor you need to know where the lowest point is in the room, the datum will show show this, now you put a
tile against the wall in this lowest spot and mark the top with a line, allow for spacing and hold another
tile on this line measure this to the datum (lets say it's 100mm),and you now know where the tiles will be at any point in the room, it's easier if you mark out these lines on a stick and use this.
Walk around the room say to the window, measure down 100mm from the datum and draw a line here, now using a
tile you can mark where each
tile will go and this will tell you how the window work, good or bad cuts.
Say you are happy how the tiles set out and you decide to
tile from the lowest point of the floor you can now mark from the top of this
tile, and draw a line around the room.
So yes you have two datums, but if you marked a full from the floor and drew a line round, you could find you've picked a high spot on the floor, so you'll get a tiny cut somewhere else, it could do the same on the ceiling, or you maybe have a cut on the window or bath that you don't like. So if you try another way, draw the line and that doesn't work, you now have two useless lines.
Draw one all around the room at any height, gauge the room, and then draw another if you like where you are sure the full tiles will be.
I hope this explains it better!