Girls do not like six pack abs. But they like a healthy male. Six packs is a sort of health definition (read fit) now a days.
Myth of six packs : Six packs is essentially starved packs!. Whenever your body fat percentage goes below 10%, six packs are visible. This is not very healthy. We need about 15% body fat to look best. No one has(should have) six packs all around the year.
Women like strong men and for that you must be fit and healthy. Sub consciously women select men based on their choice of offspring.A healthy male would give them better children and can look after them well.
Muscle are visible :Why you get attracted to a curvy and sexy lady and not a fat chick. Because you can see it. You can’t see some one’s wit from a distance. Muscles are visible from a distance.Also muscles prove hard work on man’s part. A well defined body needs sweat and hard work. It relates to your character.
Between two boys who are similar in every aspect, muscle can be a differentiation.