When you hand sew leather you can use threads made of a lot of different materials. Nowadays most people use nylon or polyester threads because they are more accessible, cheaper and also in some people's opinion stronger or longer lasting.
I however prefer linen threads. That being said I also use both nylon and polyester thread in some cases, I'm not holier-than-thou. I prefer linen thread because it's a natural material - like leather - and it was the thread of choice for saddlers and leatherworkers for centuries, which is enough for me.
Some time ago I came across a good deal on some thinner to average linen thread, which I couldn't help buying, although I've never really thought about using thread in other colours than white, natural, brown and black. But now I also have red and a blueish grey in the assortment and I have to say, that it's quite nice.
To be honest, I think, it was the nice packaging, that made me buy the thread. But it is good and it makes very nice stitches.