Precision.
Profile you want. Length you want. Calibre you want rather than factory offerings. Chamber cut for your preferred coal/bullet/bullet weights. Fluting you want etc etc.
It is most valuable when it is something you cannot buy factory.
I put a 6mm barrel on a rifle last year chambered specifically for the 107gr SMK. 28" so more steam, big target pipe, fast twist. Not going to get that from Sportsman's.
Spoke to the same smith about a longer #5 profile for my Howa 308, he said buy another rifle. He got me the Bergara Ridge (270) with a 24" #5 which did the job.
So depends, if you can buy another rifle that does the job for the same money (debatable - another scope, mod rings etc) great it will not shoot as well as a better barrel but does it matter? If precision, downrange velocity, profile, twist and the chamber specifics matter then you are into new barrel territory.