There are very strong reasons for wanting to lose weight and get fit, based on health reasons. I see absolutely no reason for a person working at a desk job to develop massive muscles which are literally only used in the gym.
Taking the deadlift as an example, Rippetoe spends pages describing the proper form, and why you need to do them. This is not because he wants you to necessarily get 'big', but because the muscles involved are used on a daily basis.
"The deadlift is more functional in that it’s very hard to imagine a more useful application of strength than picking heavy shit up off the ground. "
http://startingstrength.wikia.com/wiki/Wit_and_Wisdom_of_Mar...