Purpose: It's a safer way to park by making your exit from the spot not a backing up into an area that's tough to see. Also imagine a situation where a large number of people needed to leave in a hurry and everyone had to back up to get out. That would be one hell of a cluster f**k. Backing into a space is easier than backing out of a space as you've already cleared the area of travel before you start backing.
How is it inconsiderate?