What's the primary difference between branch instruction and branch with link instruction? ARM?

Question

From what I read, the branch with link is used to perform subroutine call and the registered link is copied back to PC.

Solution

Branch with link BL copies the address of the next instruction (after the BL) into the link register. The branch instruction doesn't. BL would be used for a subroutine call, so when you want to return to where you were you can branch back to the link register.