I think you just need to know that these people are losing weight as a job. They're on super low calorie diets and exercising 6-8 hours a day. It's nothing that a normal person can do.
I had never watched the show previously but started to right around when I started my latest 'diet'. I felt that it really helped me stay motivated during some rough patches and it was nice to compare some of the percentages as on my highest weeks I matched some of their lower ones. (It is insane to try and compare pounds lost as a lot of these people were double my size to begin with).
Now that I've gone through all the seasons I started watching the UK show Supersize vs. Superskinny to keep me motivated.