To my way of thinking...
A Pegbar is the equivalent of a Null in 3D programs if you are familiar with such usage.
A Null being an object that doesn't do much of anything except allow itself to be a place setter and reference point for other objects.
The primary purpose of a Null in the OpenToonz context is to act as a parent for other objects.
When we move the parent/Pegbar the other objects move with it.
I haven't delved deeply into Pegbars to determine if they have special properties beyond that of a typical Null.
More practically...
The specific usage of a Pegbar very likely relates to classically drawn animation but with a few digital updates that allow virtual manipulation that would be harder to achieve with real world pegbars.
In classical animation however the pegbar is essential in establishing and maintaining a reference point so that anything placed on the pegs will stay in registration.
If then under the camera the pegbar moves... so do the drawings that are placed on the pegbar.
This is particularly useful for planning the movement of characters and backgrounds on separate levels in front of the camera.
If for instance, a character is walking in a scene where the background moves both the character and background can be placed on the same pegs (in the case of the character moving through the scene) or on different pegs (if the background moves but the character stays in the center of the screen... giving the illusion of the character moving through a larger environment.