Can anyone help me -what's the difference between a ground sheet and a tent footprint? Also:

How can you pitch a rain fly without needing to put up the tent in most designs anyway?
Is it really useful to have grommets in the footprint? What do you use them for?

Assuming you use the grommets to set up a shelter (e.g. over you), how often can you do this anyway? (surely you'd need handily placed trees and appropriate fasteners such as long enough bungee cords or rope). And wouldn't the footprint need to be pretty big to give you a reasonable amount of shelter (thinking that most people have 1-3 people tents)? And then of course, if you use the footprint as a shelter it's not protecting the bottom of your tent anyway....

Assuming you use the grommets to somehow secure the tent, why do you need to? I use a groundsheet -it certainly seems secure enough under the tent.

Honestly not trying to be argumentative here, just can't work out why the foot print is worth it -am I missing some advantages? FWIW, I didn't even use a ground sheet until my last tour, when I did take a plastic sheet I'd cut to the size of my tent. To be honest, I wasn't bothered about getting it perfectly sized as if heavy rain is expected I dig a trench anyway. So why did I take a sheet?

i. it's very light
ii. it's something to sit on or put things on
iii. it protects the bottom of the tent as well as my airmat
iv. when I put my tent on my front rack, I can wrap the tent in the sheet -tent is now waterproof so in heavy rain, I know my tent will be dry.
v. doesn't move under the tent

Advantages seemed to outweigh the disadvantages -plus I did happen to camp on some very gritty campsites, so I was happy to have it with me.

Anyway, I admit all of this is a moot point, my tent isn't made anymore so I can't see me buying a footprint!