I don't know about arrangements, I still never used them. Also, I'm not sure about any of this since I also think the saving procedure is a bit confusing. But here is how I think it works:
Parts: In theory you never need to save a part, the last changes you made are always there linked to their bank and project. You just save it so you can tweak/change it and then be able to come back to where it was (by using "reload part").
The
project also does not exactly need to be saved (once again, in theory) because it is always there like you left it, even after turning the machine OFF; the thing is: I think it is in RAM memory, so "sync to card" is getting the project from the RAM to the Card so if your machine freezes (like it's happening a lot in this new OS) and you do a Reset (cleaning your RAM) you will loose everything you made since your last sync (I learned this the hard way).
Saving a backup is like saving a part: you don't really need to do it, but it's useful if you have your set ready/perfect and then you tweak the hell out of it in your jam-session and then you want to go back to where it was (restore to previous backup
)
I think this is why they don't have a "save project" like normal software, because everything is always there like you left it in your machine's RAM.
TL;DR: You just sync it so you don't loose anything in dramatic situations or when you change your CFcard; so you actually should sync often. The restore is just so you can mess up with your project and then go back to where it was
I hope this was not too confusing
and not too far from the truth