Back in 1938, Otto Fenichel developed the concept of “narcissistic supply,” which is anything that builds up and confirms the narcissist's superiority, grandiosity, and entitlement. You are the abusive narcissist’s source of "narcissistic supply." Children, especially young, are perfect suppliers because they cannot escape nor fight back. In fact, I once encountered a narcissist who was on a mission to procreate so that he could have his own child to satisfy his need for narcissistic supply (he had absolutely no interest in the accompanying partnership or love relationship that is the basis for parenting).
What are you supplying?
Attention, both positive and negative. Adoration is obvious. Causing you problems, and thus being at the center of your world, is not so obvious but just as powerful.
Believing in the narcissist's accomplishments, even those that are imagined and exaggerated.
Sex, and proving their masculinity or her femininity.
Leading a stable life, which the narcissist would be unable to achieve without your help.
Security, in the form of a home, money, and family life
Reputation, belonging to an organization, some element of fame or position, success, and owning property. If you are "something," you erase their "nothing."
Status, including your physical beauty, position in society, or other intangible symbol.
Scapegoating. You provide the vessel in which the narcissist can pour their never-ending flow of toxicity.
You merely function as an object to supply what the narcissist needs.
May I also point out that this means the narcissist does NOT LOVE YOU in the way you need and want. The narcissist only NEEDS you for what you can do for them.
Finally, your narcissist doesn't want to lose you because, quite simply, they hate to lose and losing means giving up, losing power, and reflects their inability to remain dominant. It means losing the source of his supply, and leaving the narcissist vulnerable and often without a place to live, without income, without food. If you do leave, the narcissist will fight to get you back -- so that he can continue to get what he needs.
Are you ready to cut off the supply?