For me one of the biggest differences is the ability to do things in bulk. I can have a whole set of photos that need some basic editing such as changing the color balance and bumping up the blacks. I can do them all at once in LR. There may be a way to do it in bulk in PS, but not that I've found (granted I'm a PS novice). I also find the crop functionality of LR to be quicker and easier. I own them both and do about 95-98% of my work in Lightroom.