Even if you have a piece of equipment at home, you might opt to rent it at the location.
Firstly, local rental shops know the sort of equipment that's required to safely and comfortably do the activity for which the region is known. Provided you don't go to a dodgy shop basement shop that's trying to offer a ridiculous bargain, you should find informed sales staff who are able to explain and direct you towards what's needed for your particular adventure.
Secondly, renting is a great option for expensive items that you're unsure you'll use again. If you're camping in a cold climate for the first time, for instance, you don't necessarily want to buy your own four-season sleeping bag. You want to first see how much you enjoy the experience before considering making such a big purchase!
Thirdly, renting means you don't need to transport certain items (which are often heavy and bulky) across the globe. Even if you have a piece of equipment at home, you might opt to rent it at the location.