It's like selling an item in classic LoL. You do it when you think the shift in strategy will be even more of an advantage then the plus in 'value'. So you only do it when your strategy is not working or you are super late and you have lvl2 champs on the bench for example. As in classic, they try to make you think mid-long term strategic, which includes not being able to reroll every decision you make every round. I was surprised by not being able to change items at first as well, but that's clearly intended by riot. Just a matter of time until everyone is used to it. In classic you can't change items as well. Also funny, but no one asks for it anymore.