You definitely can use it for leg space, or rather, for knee space. You're not going to stretch your legs into the isle, but just poking out your knee a bit can

You definitely can use it for leg space, or rather, for knee space. You're not going to stretch your legs into the isle, but just poking out your knee a bit can give much desired relief for those with long legs. For me on a budget airline, this tends to be worth it. Even more so if the person in front of me decides to recline their seat.