To be clear, sidechain transactions are just transactions on another chain. I wouldn’t call that off chain as it actually introduces more transactions (to move across the peg).
It's less transactions. Imagine you're paying your cable bill.. you can move some value to the side chain (one transaction) then make 12 monthly transactions-- on the side chain, so off the main chain--and then after cancelling move the balance back to the main chain (second transaction)
The amount of on chain transactions supported by a global mining network is fixed. You can’t just keep adding side chains or else you will run into the same scaling limits.
Thus 14 transactions become 2.