The soul of an individual is made up of two parts: The ghost, which is eternal and lives on after your death, and mana, which is the life energy that keeps your body functioning and your soul grounded to the mortal realm. When a person dies, their ghost travels to the hereafter while their life energy dissipates into the background of the world. All people are born with a set amount of mana, and there is a process in which one can steal theirs and add it to their own. This has led to human trafficking rings developing around the world in which people are kidnapped and sold to individuals interested in this practice.
For this procedure to work, ancient runes must be carved into the flesh of a living victim from head to toe. Once this is done, the person is killed in a sacrificial ritual. This allows the ghost to leave the body, but keeps the mana contained within the corpse. From here, the witch/wizard can do one of two things. They can seal the reservoir of mana into a containment jar(s) for later use, or absorb the mana themselves.
Sealing the mana in these specialized jars for later provide a temporary boost in power for spells or rituals. However, a significant amount of mana is lost upon the transference process from the corpse into the containers. It is also hard to store, as the mana eventually erodes the containers and decreases in quality over time. Absorbing it into your body immediately adds a permanent boost to your supply and makes your magic much stronger and can even extend your lifespan. Even though people are born with a set amount of life energy, the body can hold an unlimited capacity. All of the victim's mana is absorbed upon this process, with none of it wasting.
Why would the 1st option be more attractive to mages if it provides less returns on the investment?