Arduino's ground

I’m soldering up a proto board, and I was wondering if I can just solder the negative lead from my battery pack to the ground on the proto board instead of soldering it to a 2.1 mm plug. I tested the negative side of the 2.1 mm plug, and found it connects to the ground on the prototype board I’m making, so it appears I can do this. But I’m not sure if there is some internal protection or reason to apply power to the socket instead.

can just solder the negative lead from my battery pack to the ground on the proto board

yes