Sometimes it is simply because you can or you want to. I have a 22" fast twist on order for a 22 Grendel I am having built on a Howa mini action. I have never used a carbon barrel and wanted to try it. They are obviously light for barrel contour, but at this point I am so far past need, or reason for that matter, in the rifles I buy or build that I am simply having fun and seeing how stuff works.
My build is a M40 taper, M40 stock from B&C and a 7 twist. My plan is Hornady 88s or Berger 90s for now and Lapua brass. Not positive on powders yet, have some reading to do and does to get ordered.