I don't know if many people know about this, but it was introduced recently with patch 7.1, but seems to have slipped under the radar, at least for me. I only discovered it when I accidentally stumbled upon it when messing around with the glamour dresser.
Basically, when you have matching sets, you can store the entire set using only a single slot in the dresser, rather than a slot for each piece. Currently, only some sets are supported, mainly special MSQ rewards, store sets, PvP (Wolf mark) sets, etc.
When you access the dresser to store items, outfit-eligible items will have a special button next to them in the list. Click the button to show all the items you need. If you have the full set, follow the instructions to store it as an outfit. You will then be able to use each piece individually as glamour; you can still mix and match like you did before.
If you already have a full set of outfit-eligible gear stored individually in the dresser, but want to store it as an outfit, you will have to remove each item and re-insert it as an outfit, costing a glamour prism for each item in the set.
A full explanation can be found in the in-game help. Click the "?" button in the Glamour Dresser window, and all Outfit Glamour related entries will be in "The Glamour Dresser" section.