When working on a project, its obviously important to use the right resolution
Well, it's important to work at the correct size, although arguably not that important*. Work in vectors for anything and everything that isn't specifically an image, then resolution doesn't matter (vectors have no resolution).
It's important to use the right resolution for assets. So your placed photographs or background images or whatever certainly should be at an acceptable resolution, but a whole project in Photoshop (or equivalent) shouldn't really happen (unless you're working for a screen output, where resolution isn't as much of an issue).
Ok, so maybe I'm being pedantic but I thought it should be said.
why can't you always use a high PPI for all your project and just scale down when you only need a web version of your file.
why can't you design a A3 poster and then scale it down to A4 if needs be?
Again, you can!
I often design posters that are used in multiple sizes. I can design at A3 and have the exact same file used to print A4's, A1's, A0's, even A6 flyers.
As Alex said (and is very much correct in saying so), that isn't really the ideal thing to do but sometimes you only have a budget for a single poster (and working within constraints is just part of being a designer). And as I said, working with the correct tools (i.e. vectors) means it's only the linked assets within those posters that need to be of a certain resolution.
* The beauty of A sizes. I can work at any A size I like, and output to any A size I like; and resolution probably isn't as much of an issue here as you'd think — you can generally get away with printing an A0 poster at a much lower resolution than an A6 flyer.