Lots of reasons...
1. to be healthier of course. I want to live a long life that isn't shortened by being obese or just suffering through the last years in a sedentary state. I've seen it and it isn't pretty.
2. Vanity...I want my teen boys to never be embarrassed of their mom. When their friends go home, I want them to say man his mom is hot.
3. For my hubby. He loves me just the way I am but I see him looking and touching more often and in a different way than recently when I was so heavy.
4. To be more professional. I know that looks aren't supposed to affect your job, but they do. If I were to apply for a position and someone smaller did with the same qualifications. They would get the job.