Ok, here's the pics:
4 panels (8 tris) with a gap:
Same with overlapping, unwelded seams:
Everythings looking honky-dory but the thruth is much nastier. The next picture shows how GPU has to remake new tris to complete it's task. It also shows how modeller need to the seams.
The backside shows the other side of the picture, literally. There are a lot of thrash. With welded seams, all those faces that are not showing at all, they are "deleted" from the package that gets sent to GPU. This is called
culling and CPU does that for us (note! culling is done BEFORE everything is sent to the GPU for the picture to be drawn on screen...) Following picture represents the endresult with unwelded seams and after GPU has made all the new tris. The GPU does not do any culling after it has made all the corrections so it has to draw those unseen tris...
This is how the underside looks after culling with proper welding.
When you think that after GPU does all that new geometry, it has to apply those changes to UV mapping (how textures are mapped) with multiple shader maps, how HAT has to consider multiple materials intersecting and their feedbacks in that transition.... We end up with A LOT of unnecessary processing, lowering the overall perfromance in ALL levels of the process. Mesh filesizes is the only exception, that of course grows slightly but comparing it to the amount of unnecessary realtime processing....
So that's why the banking needs to be done with only one mesh instead of three. You save on everything in that case, file-sizes are smaller, fewer individual meshes, CPU power, GPU power, less RAM/VRAM usage etc...
We don't need to be concerned with objects going thru the ground, GPU has enough power to deal with that plus i think it has some nice methods of streamlining the process. But since the road is the most important part of track creation, it needs to be done optimally and with minimal amount of inaccuracies. There is some occasions where collision detection goes haywire with the same problem; unwelded collision surfaces where the object can shoot up in space (or gets stuck, pushed thru the ground etc) when it receives multiple collisions simultaneously happening in different directions.
Most of us bumb in to this phenomenon in some part of our "career". Most often it happens with sobjects, CK-Pits_and_barriers XPack for ex has some conditions where the tirewall does that (follow spline, drop object to follow the ground = stepped sobjects does that with properly made objects very often, even the Default XPack objects can be made to do this.....)