Thanks to previous occupants of my house, one of the bathrooms is lit with a pair of lights that are unlike anything I've seen. The bulb is a narrow cylinder, about 3 inches long and maybe 1/4 inch in diameter, with contacts on either end. The middle is filled with some dark material, and there is a tiny, clear protuberance in the center. It is marked "PHILIPS CHINA," and "130V 2E 150." Below are pictures of it in place and out.

What is this fixture/bulb called?

Why would anyone install one instead of a normal fixture?