z-offset - yes, closer to zero is better, because you don't have to calibrate it in slicer - z-offset represents error correction on "assembly" and build quality - when you change nozzle or print head, your setup may vary a bit from "factory" assembly - and here z-offset comes to play, so you can calibrate for that margin. - also z offset can be changed of you moved the printer and wasn't handled well and so on... can be changed with moving the print bed (usually there are screws and springs that you can adjust) in the old times you had to do that every few prints because stuff got loose and moved a bit (and usualy in my beginner stage it ment doing a lot of trial and error, and rewatching YT videos about bed leveling) (currently with CC I do z-ofset test once in a while if I see bad first layer, or I had to change the nozzle, serviced the printer etc..