Mostly, it's the pred's domination over the prey. The pred can do anything to the prey, effectively "owning" them. While the prey becomes helpless and simply reduced to nothing but food. This is an ultimate form of domination. A party having full control over some helpless and powerless being. If the vore is fatal, that feeling is amplified because now it's effectively about the prey's existence. Not just physical being.

Depending on whether the prey is willing or not, this can put a spin in which the prey intentionally put themself on the same insignificance as food, and acts accordingly. Willingly abandoning their worldly happenings just to feed someone. Because the latter is more important to them.

For the prey, it's not always about domination. Like, maybe it's just the experience of melting or liquefactiophilia. Or being so intimately close to someone that they choose to literally become part of them. Maybe they just want to be turned into another form via digestion. Sometimes mistakes or accidents feeds into the allure of inevitability. If you ask different people, they will have their own interpretation and preferences.