As far as weight loss is concerned, "healthy" food helps insofar as it makes it easier to eat less and exercise more, which is a property that some (not all) healthy foods accidentally happen to have, and which they share with a variety of less healthy calorie sources.
1. Why exercise if you never plan on engaging in vigorous hobbies or professions?
2. Why seek physio therapy for a stiff joint if you spend your days on the couch or in a chair?
3. Why see a dentist if you only receive calories from drinking soylent?
4. Why bathe if you never leave your house?
There's some philosophy of morals argument to be made here similar to the concept of whether it is good or bad to eat arsenic.
1. Because it's healthy and feels good
2. Because it's healthy and may stop future problems
3. Because it may stop future problems
4. Because it feels good
So why lose weight for the sake of losing weight? It's neither healthy nor fun if you do it the wrong way.
Because chronic obesity is associated with numerous morbidities.
The same answer you gave for (3) applies to avoiding obesity.
Many possible reasons. You would be surprised at the number of people doing it mainly to be able to fit into pretty clothes.
It's difficult to find motivation to lose weight to be healthier when you don't actually have any health problems yet.