Samemaki literally means "wrap same(gawa)." When working on the saya, it's the process of wrapping the ray skin around the saya. In some very fancy cases, people have wrapped the entire saya. However, it is more common to see the saya wrapped between the koiguchi and the kurikata.
There are a few main reasons to do this procedure. The first is that many people simply find it attractive. The pictures displayed here show the samegawa with a kind of distressed effect, but it is often done in full black as well. The same is generally sanded down and lacquered so it provides texture without being "grippy" in the obi.
Second, this serves as a great repair for a saya that is cracked near the koiguchi. It looks good, and samegawa is very tough stuff. It will fix a crack or minor break in the wood and make it stronger than it used to be.
Finally, it's great insurance on a shinken, especially for those just beginning to work with a sharp blade. In iai practice, the koiguchi area is often held by the left hand while the sword is being drawn, and if the swordsman accidentally starts the cutting maneuver while it is still in the saya, things can go badly. The tough samegawa gives you an added layer of protection.