Just another opportunity to drain resources out of the system /shrug. C'est la vie!

While rhetorical, I think it's a fair question to ask.

So cost of it aside, I think the biggest reason why it does and should exist is Arena. It would be pretty rough and make it kind of a nightmare if players could just swap around their best gear for free constantly in Arena. 

One of the important things about the game is players being needing to make decisions that matter, and where we allocate our gear is one of those decisions. 

Because all equipments melted on your body and you need pay for a doctor to remove them or all is cursed for the good RPG story, but i dont know where is the temple for cure... ;)

It all makes sense finally... They didn't want us to know because that is what the Void Tower is going to be! It'll be the temple to cure the curse!

If you could swap the gear for free, you would need only one setup for damage dealer, debuffer, reviver and so on and could swap the items among all your teams. So you need different gear for all your champs, and if you look to faction wars, this will be a lot of gear in later stages of the game.

It makes sense to have a cost, but it should be lower.  Either that or they should have a freebie day once a week.  Or maybe make it so you could swap, say, 45 pieces free every week (i.e. one whole five-member team) before the silver starts ringing up