For me, I think it's a combination factors.
I think it gives a bit of peace of mind knowing exactly how that rifle was built, by whom it was built and the what parts used, generally it's pretty exciting / enjoyable ordering parts and having them show up. It's like Christmas everyday.
A lot of it is creature comforts. Having a rifle built exactly the way you want it, and having max adjustability in the areas you want it. Also, time. These newer aftermarket actions built with such close tolerances that you no longer have to send your rifle off for sometimes months on end when it's time for a barre swap. Now you can order Prefits for your rifle in the material, contour, rifling and caliber you want. To me thathuge.
my daughters a lefty, so I didn't want to have to build her multiple rifles depending on what she wanted to shoot, so I built her rifle on a Bighorn Origin SA and then got her all the bolt heads for $125-$150 each (compared to $400+ for a replacement bolt) . Now she can shoot .223 to train, and anything up to a short mag depending on what she's hunting with nothin more than a barrel and bolt head swap. Right now she has 16.5" 6.5CM, 22" 6.5PRC, and a 22" 7SS barrels for her rifle.
That last factor is one of the biggest for me. What's your time worth. I hated sending my rifle off for weeks or months on end for a barrel swap. IMO, the cost is worth it to be able to order already chambered / threaded barrels and install them yourself. Keep in mind, I'm some cases (bighorn origin @ $800) you're gonna spend the same money tto get a Rem 700 and then true it up and dialed it in as you would for that custom action and you still won't have pre-fit barrels available for it or the ability to run different calibers without using full replacement bolts.
atleast that's how I try to justify it….