The caulk hull simplifies the VIS process for the compiler (the VIS process being what is visible from where so the engine knows what to hide from the player when he's in a given location).
The VIS process breaks the map up into blocks (called "leafs") starting with 1024x1024 sizes that can be seen in your viewport when you turn Show Blocks or something on. The compiler then checks the sides of these "leafs", called "portals" and decides which portals are visible from each other, taking into account Structural brushes blocking view. If a portal cannot be seen from another due to a structural brush being in the way, it won't be rendered when the player is in that location.
It then keeps breaking the map up into smaller blocks or something until it's happy it has tagged all portals that can't be scene from a certain area to not be rendered when the player is in that area. This is why a simple caulk structural hull is good. If every torcholder, button, and railing were structural, the VIS process would take a long time and I guess, not do too good of a job. The reason some people make walls and floors detail is mostly to avoid overdraw (a textured surface being overlapped by another) which causes z-fighting (that visual jaggedness seen at some scenes when viewed at a distance, worse on some cards and bit modes than others).
If the compiler only needs to VIS a map made of squares and rectangles, it would be much easier on it, which is what the caulk hull is since all detail isn't taken into consideration.
Now, the VIS process being a bunch of 1's and 0's, it's pretty stupid. This is where you can take hint brushes and patch the VISing, in effect, manually adding leafs (the hint brush itself) and portals (the sides of the hint brush). If you have a 90 degree right angled hallway that is being rendered beyond what you can really see in game, you can fix it by placing a triangle hint brush in the corner with two sides flush with the outside corner of the hall and the third side touching the inside corner (Shadowspawn has a screenshot up somewhere that illistrates this). The visible portal of the hint brush is the side that is bisected by the inside corner of the hallway. If you were looking at this portal from outside the hint brush, it's impossible to see the other half of the portal from where you are without entering the hint brush, due to the inside corner of the structural hall. So anything beyond that portal around the corner will not be rendered until the player enters that hint brush.
I'm almost positive I'm wrong on the technical details but that little bit of understanding has really helped me fix VIS problems.