Why? So you can put a nice edge liner on a piece of sheet metal like this:
Here's how I did it.
I took a piece of 2x4 and drilled a hole through it slightly larger than the tube I'm going to pull through it. It'll need to be the width of a saw blade larger in diameter. The hole needs to go through the thin edges and be at a 45 degree angle from the flat side. See pics below and you'll figure it out.
After that you'll want to cut the block length wise as shown. Make sure to split the hole 50/50. It's kinda not easy with a circular saw... use a band saw if you have it.
Now crack it in half and get a razor blade. Position the blade so it cuts halfway through the hose. Drill a small pilot hole though one of the notches in the blade and two more holes along the sharp end. You're using screws to hold the blade in place. three of them. (I was able to cheat...).
Now put the block back together and finish the pilot holes going into the other half. Open it back up, start the screws so the tips just protrude into the cut area, place your blade and screw both halves together.
Now you can push the hose into the end and when it pops out the other side it'll have a slit in half of it.
Here's the finished jig
...and this is what I did with it.
Just got done making the filler panel, plasti-dipping it and putting the finishing edge on it. I think it came out pretty nice and it only cost me about $20 (I had the metal already). Saved $120 over buying it.