The assumption is that there is not much room in the lane. When you pull in forward, especially with a longer vehicle, you need more room to swing out and get the front end aligned with the spot before you enter the space since the rear just follows the front turning wheels. When backing, you just have to get one of the rear wheels into position and then the front end swings out while pulling into the spot rather than before pulling in. It’s way easier to pull out of the spot when you do this, too, because you can turn the wheel immediately, whereas when you’re front in, you have to back almost all the way out before you can start cutting the wheel. Of course it also depends on how far past the rear wheels the vehicle extends as to how much it will swing out.