Wraiths are the response of the world to the dangerousness of emotional energy, expositing from their viewpoint that emotions have the potential to overturn and rewrite the universe, and that the fabric of it shudders and recoils from the possibility, birthing them.
They respond to strong concentrations of emotion and devour them in order to digest them into the safe form of Grief Cubes, so the spawning of them is literally 'You kill a Wraith and collect what was in it's stomach'. The consumption of emotions destroys them and the memories attached to them. If Hitomi's love for Kyousuke was devoured, she wouldn't remember loving him or why she loved him, if she remembered him at all.
Because magic is emotional energy weaponized and concentrated, a Wraith can steal your powers. A Magical Girl can have some of her magic eaten so that her Soul Gem, rather than being stained black, becomes darker in hue even if perfectly cleaned. Powers are lost and your MP pool's maximum shrinks. Kyubey claims that he's never seen a Magical Girl get her powers back by defeating the Wraith who stole them.
The Wraiths concentrated around Homura in such intense amounts because her power was frighteningly large, the presence of Salamander imagery implying she had her AI YO potential ever since she entered the Madoverse. When her Memory Magic was stolen, the Wraith that did so wasn't able to digest it, and so smaller Wraiths kept trying to siphon off parts and digest it, causing them to turn into humanoid doppelgangers people's memories, like a Sayaka who appears to Kyouko and Hitomi after Sayaka's death.
Things come to a head when a Wraith takes Madoka's form, appears to Homura, and explains it doesn't want to fight. In fact, it wants to help her get her magic back, to keep its race from being mutated any further. The clones aren't acting as Wraiths should. This one included, its bump in sentience making it act like the sweet Madoka Homura longs for. It knows it.