They exist in the same way markers on a map exist, or a diagnostic criteria exists. It's something you use to navigate and make sense of things. If it doesn't work for you, no shame in throwing it out.
If I were asked to pin them down...I find them far easier to understand when you differentiate them from their opposite, so:
Se: Experiencing things in terms of their "sensate experience" as they happen to you, directly, in real-time. No filter, no greater awareness, just what it is.
Ni: "Looking at" things in a way other than their direct reality. Abstracting and experiencing things in terms of how they fit prior abstractions. Wholly re-contextualising what is happening.
Ne: Experiencing things in terms of their abstract connections as they introduce themselves, and what that might change. Imaginative, broad and adventurous.
Si: Relating things to how their "sensate experience" matches up to prior sensate experiences. Extremely grounded and specific.
Te: Quantifying the world as it occurs, so as to more easily navigate and exploit it.
Fi: Understanding things in terms of innate desire and need, as it emerges from something's actions or expression.
Fe: Aligning to the world emotionally, navigating in terms of creating an affiliative and political harmony.
Ti: Understanding things in terms of a logical, timeless "wholeness", watching the clock's gears move everything behind-the-scenes.
Michael Pierce does a great job of this: