Restless spirits are troubled and spend long times within the spirit realm- Some cannot move on so rapidly, oftentimes the souls of thosewho fell in battle yearning for some final encounter, one where they matter.
The magic art of spirit summoning provides catharsis for the restless spirit. Unable to communicate and often already stripped of their memories beyond vague emotions, spirit summoning is the ordinary method by which humans and the dead co-align to further the rebirth and end the fetters of combat and regret that keep a soul from beginning a new life.
There are some more perverted methods of holding and contracting spirits, some benevolent ends such as a long standing ancestral grave, the pact of the old Exorcists to fight evil wherever it may arise at the cost of incarnation until they are satisfied, as an example.
Depending on the strength of a soul and the emotional strength of the ties that bind it, a spirit may be able to reform itself over a period of years to manifest for a summoner once more.
A spirit is always merely a shadow of it's former self as the stars carry on their own work, the magic as it works as intended seeking to merely fulfill the ghost. Rarely are powerful magi able to be summoned, and would probably be unrecognizable as called- Oftentimes the emotions that bind them and their power is simply deemed too dangerous by stars to release back into the world even for catharsis.