Benefit of Item Sets? - Elder Scrolls Online - ESO

Before Summerset was released in 2018 it was not possible to get Jewelry for crafted sets, but with that chapter (which is now a DLC) Jewelry Crafting was first introduced. So, aside from the jewelry that can already drop in the game from Overland, dungeons and trials or is being sold by various vendors, you are now able to craft jewelry for gear sets that can be crafted.

Additionally you can now upgrade the quality of a piece of jewelry and change its trait at a Transmutation station, if you have the trait researched.

Note: You can learn more about Jewelry Crafting in the Jewelry Crafting Guide.

The picture below shows a set that can only be obtained as a jewelry set, the Willpower set. The necklace has the arcane trait. You can find a full list of all traits in the "Pick your Traits" section.