What is Nozzle Offset for?

Question as the title!

Not sure what it should be and when it should be changed.

That setting changes the height of the nozzle relative to the print platform.  A negative number moves it away from the platform and a positive toward it.

It's a quick way to print closer or higher from the build plate. In my case, when i dont use a raft, I add a .2 and it sticks fine and doesn't click. I leave it at 0 when printing with a raft.

Interesting, I thought the printer would adjust by itself when printing with or without a raft, that might be the reason it failed when I tried without a raft ^^

I would assume it has nothing to do with the raft, but rather just a simple offset from the build platform. I've been using it with my PETG and Wood PLA prints, and it has lowered my stringing slightly. Always with a raft.

I have a question.  When setting nozzle height calibration, they have us subtract .5mm from the height.  Is this new zero the correct height to print with raft, and we would add back the .5mm in nozzle offset to print raftless? 

I knew at one point, but forgot if that was correct.

If we DON'T subtract the .5mm in setting calibration, will a nozzle offset of 0 be for no raft and +.5mm be with a raft?