Because I can. And there's always someone angling after taking the old one off me whispering "don't you want to buy the new one" in my ear ;-). It helps that when I hand over an old phone it's like-new - I take care of my toys.

It is of course non-essential expenditure. But we're all allowed to enjoy something... Other people spend money in ways I find inexplicable (like buying or driving cars) so, horses for courses I guess.