Every artist needs to learn about the human form, and that means actually hiring a model at some point. There is only so far you can get with photographs and drawings; at some point, you need to actually draw from real life. London life drawing models allow an artist to look at an actual human being and see all that they would be missing in from a mere picture: muscles, skin tone, and even blemishes.
A life drawing model allows an artist to actually look at a human form and get a better idea of the proportions required by that form. It is great to know what the proportions are and work from that, but only by drawing an actual human can artists truly appreciate the functionality of the human form and how it works, making for fat better illustrations than from someone who has never drawn a human before.