Purpose? None. It's not a deliberate choice, no more than someone screams at pain. It's just a thing.

Same with being a billionaire. It's illogical to not want things like happiness, power, immortality etc. It just is the way humans are wired - we want things we cannot have. This is why I think it just clearly true that most people are not totally happy.

However, the desire to live longer and younger and healthier for longer has certainly driven humans to much good science, diet, medicine etc and will likely result, in time, to an end to ageing in total.