There are many reasons for lining holsters and belts. Due to the fact that two layers of leather will be more structurally tough than a single layer of leather of equal thickness, there is an advantage. Another is that the rough side of leather attracts dust and grit more than the smooth side. Many believe lined looks better.
They cost more because of use of much more material and take about 2x as long to construct. I have found a lot of difficulty finding a good lining leather. It needs to be very thin, yet tough and cheap. Thicker veggie tan results in a lot of wrinkling on an inside bend. So far thin (2oz) goatskin works best for me.
