It is called "solder mask" and the main purpose is to keep solder where they belong during SMT reflow process.
Without solder mask, the solder will spread and flow away all over the copper and either create shorts or open solder joints (insufficient solder) during the SMT reflow process.
The solder mask is not required for manual hand soldering process (home builds).
If you want to prevent the bare copper from oxidizing, just spray a couple layers of thin coats of clear lacquer once the project is completed.