Summarizing Rarity
Rarity shows how hard something is to obtain or how much are out there. News Paper for example would be a "Legendary" or "Discontinued" Rarity. It can be used to unclog the inventory and make it nice looking, ofcourse a good use for rarity would be a Sort Table feature. Sorting tables can show different rarity, and maybe other cool features like the date obtained (Unlikley saved though so most-likley not getting added). It can also use for a nice interface.
Why Rarity?
Rarity seems like an odd choice yes, but there is a bit of usefulness. Like I said above, you can "Sort" items based on rarity. Rarity may put an impression on players to buy skincrates, to get a "Legendary" skin. You can sort Event items, being the higher the tix the more "Rare". This Can also be for the "Usefulness" of a weapon, armor, or even a mount. It also makes the UI look a bit colorful, you can also hover over the Rarity Icon to give a text saying "Rare" And Maybe how many people own it: "25,364 Owners".
What will Rarity look like?
I managed to make a few images that look nice, and I think I have a good selection below:
The List Of Rarity*This can be edited*
C (Gray) = Common
U (Yellow-Gold) = Uncommon
R (Blue) = Rare
E (Purple) = Epic *by epic games*
L (Orange) = Legendary
-- Special --
D (Greenish-Blue) = Discontinued (Aka events too)
M (Red) = Moderation Restricted
Example Below: