I use Item Sets as a mean to organize myself prior to. I am one of those players who analyzes everything and I would like to think what works and why. I get items that can work well with my champ for various situations. For Example-- Ahri: Morell, Sorc Shoes, Void Staff, Proto Belt, Zhyona's and Deathcap. Generic items that I like. If I find that Rylai's Crystal Scepter would be better for kiting purposes, then I will add it. Or Liandra's Tourment for tankier targets as well as Magic Pen.
Or having multiply builds for a champ. Example-- Poppy: Tank Poppy, Bruiser Poppy, and Jungle Poppy. One focuses on tanking damage, one dealing damage, the other is jungling... all self explanatory. Each have items that I may or may not put in the other builds. I may put things like Attack speed on some of them as available options if I want to destroy turrets... which depends on the game.
There are a lot of items that I have for situations depending on the game and I can easily forget about the item. Item Sets reminds me of items that can work with champs. An example of this would be the red trinkets when playing support. Get SighStone and then Red Trinket. I get the Sightstone but then forget Red Trinket, usually because I am in a rush to get back to lane to get the lane back to 2v2.
I am on the new client and when Pre-Season 7 items came up, I kept forgetting to add the new items to test them and get the Control Ward over the Pink Ward. I can be forgetful while at base to get some items and only remember once I leave base. No gold left to get them. I like Item Sets, and as someone who has the new client, there is no Item Set. And I disliked it because when I was testing Support, I forgot Control Ward and test new items.
That is my reasoning....