I remember someone writing about how leg development got a lot better in general when this machine was "invented". Thought about it and it makes sense. You can place a ton of tension on the quad and you're fairly comfortable while doing it, unlike squats.

Plus squats do not suit everybody's structure. Look at Arnold squatting - not a good structure for this exercise.
Personally, after about 20 years in gyms I have never really leg pressed on a 45 degree machine. I only get a very short range of motion I'm sure due to inflexibility. Heels come up after a couple of inches unless I have half of my foot off the platform, and then it's a glute/ham move mostly.