"In reality the license just puts extra restrictions on distribution of the software and how derivative works can be licensed."

Actually, copyright is now implicit. There is NO right to distribute software and NO right to create derivative works. Copyright gives those rights exclusively to the copyright holder.

The GPL makes it legally possible to distribute and create derivative works. Without that license, you have no such right, and can be sued (for damages) by the copyright holder. Proving damages can be difficult.

In any case, the license is the only thing that allows that behavior. Without it, there is no right to do it, and thus restrictions are moot.