Set

ive heard that the rarities are unique, rare, legendary, fabled and mythic and in this way they are from worse to the best then whats the purpose of set rarity

 

well there are different set items but basically if you get the full set you get a bonus and that makes it good.
probably legendary/fabled level of strenght for sets like champion or morph

 

Depends on how good the set is
Set items give a bonus when worn together
Good sets like Morph are probably fabled or at least legendary as stated above
Set items on their own are probably unique, and half a set is rare bordering on legendary