Several reasons, actually.
In a nutshell, it's to get beer to my tower, and keep it cold on its way there.
In more detail, it's because I was using a chest freezer converted to a beer cooler, and stored under my bar. You can't cut the front off of a chest freezer to make it front loading, and I didn't want to hinge the 8' bar top to get to the lid. So I was left with a requirement that the freezer be able to slide out from under the bar. Since the tower was to the right of, and higher than the chest freezer, I had to engineer a flexible trunk line. This is to deliver the beer to the tower, and allow enough slack for the freezer to slide out. The obvious problem with this is that there is 7' of line outside the cooler getting warm! I could have bought trunk line from MicroMatic, but I didn't want to spend $7+ per foot for it. So I decided to build my own. The chiller build was for the same reason. I could have bought a proper glycol chiller, but they are expensive, loud, and big.
The last reason is just because I like to build stuff! And it was fun