Because the liner is made of softer leather, it serves to protect the gun's finish from wear.
Here's a tutorial about making a cowboy holster. The very first page contains links to books and patterns which are being sold.
http://unblinkingeye.com/Guns/CH/ch.html
There's another holster making tutorial, and pages about holster making tools and additional notes about holster making on the home page near the bottom on the left:
http://unblinkingeye.com/Guns/