Longevity and quality of the typing experience. Mechanicals are capable of something like 50 million keystrokes, whereas even the best membrane keyboards are only capable of 20 million, but you'll notice the membrane going well before the 20 million mark.
A mechanical is expensive, but you'll only ever need to buy one in your lifetime. Though, with all the different switches in various boards, you'll probably find yourself wanting to try more as the mechanical bug bites.
If you spend a lot of time writing, programming, coding, scripting, whatever at your keyboard, you want the best tool for the job. And switching to a mechanical is like having an epiphany. I got my first mech a while back and now I wouldn't type on anything else but a mechanical unless I absolutely have to (like when using a laptop or my brothers computer).