Str+ adds one permanent point to your base strength stat. If you do this often enough, you'll hit the 255 cap for stats, and can start junctioning weaker magic (like using flare instead of meteor, and using meteor for something like elemental defense)

You do not have to get Odin before the Tonberry GF, but I don't see why you wouldn't, unless you wanted to keep Odin in disc 4 and didn't want to wait that long to use tonberry's abilities.