I think it's pretty simple. I found that navigating (moving mainly or translating in other terms) in 3d perspective mode isn't comfortable enough.
In blender you can navigate your center orbit using "Around Selection" mode or just as free point. My patch is useful in second case, where you actually don't see this free point. So, this patch draws it for you!

To check how the patch works go to 'User Preferences' -> 'View & Controls'.
Then find a button 'Orbit Center' and turn it on.

Now when you're rotating, translating or scaling (middle mouse button in 3d view cleanly, with shift or with control respectively) you can see a little circle in the center of the view. This is the center of the camera orbit! :)

Also it uses depth buffer to distinguish between 'inside something' or 'outside' state and draws itself with different colors.

It really helps me navigating around when I do a lot of rotations and translations in 3d.