OK, everybody step back, take a deep breath, and enjoy goofing around with this thread. It is intended for fun.
Rambo, I loved your spool of thread, but I resemble that remark!
Danny, in answer to your question, it is a matter of reducing the chance of the vein collapsing post-operatively. Turning the vein inside out accomplishes two things that will result in a stronger blood vessel. I have prepared a simple diagram to help explain this. Keep in mind that the cells in living tissue are basically balloons of water, furthermore fluids such as water cannot be compressed.
The diagram shows the cross section of a simple vein, comprised of eight cells (red) forming the inner wall of the vein, and eight cells (blue) forming the outer wall. Note that the cells of both the inner and outer walls are the same thickness, however since the outer wall will have a larger circumference than the inner wall the volume of the cells in the outer wall is 1.4 times that of the cells of the inner wall. When the vein is turned inside out these larger cells (red) wind up on the inside, and since fluids cannot be compressed the cell is forced to reshape itself, becoming thicker, and being thicker, more rigid and stronger. The cells that had originally been the inner wall (blue) are now the outer wall, they now however have to stretch themselves in order to form the outer circumference. Since their volume must remain the same they become thinner. This brings us to the point where we must understand that the cells have a certain elastic memory, and will attempt to return to their original shape. This elastic memory cause the blue cells to act much the same as shrink wrap, holding the blood vessel together and stabilizing it.
Does this answer your question?
Everything that I have written here is made up, I have no idea why.
TW