One can use whatever reel seat you wish of the proper inside diameter on the rod. You can use whatever color thread you wish. You can shape the cork to be exactly the way you want it to feel in your hand. You can use whatever guides in whatever plating you wish. You can add feather inlays, or weave your name in thread, etc. on the rod.
In short, you can have things on the rod that the factory won't bother with doing or will charge you quite a bit more for adding to the rod, unless you get the rod from Meiser or Burkheimer because they build custom rods on their own blanks.
Other than the above, there is absolutely no reason to build a rod from a blank. Just like there really is not reason for most hunters and shooters to load their own ammo. Or for a hunter to make a stock of his own either from scratch or from a semi-inletted or finished blank.