1) When starting off on a map, use Dev textures to "size out" the map, make sure you get your rooms to scale, etc. This makes texturing an after-thought, meaning you can focus on brush work. When you come to texture your map, you texture all the visible surfaces. It is VERY easy to spot a dev texture, and therefore know if you need to give a face another texture. Once complete, one can simply replace Dev textures with NoDraw.
2) Optimisation. The maps that use heavy Dev textures usually are gun games like Aim maps, etc. For these, FPS is key, and to make things load quickly, everything is textured with these simple textures, so that less textures need to be loaded into the RAM.
For what you mean "why do i see it so much", it's probably number 2. However, number 1 is important for good mapping.
you can easily SEE a dev texture, and therefore know you've missed a face to texture. You can't (as easily) see a nodraw texture, and therefore more likely leave it in game, untextured.
Oh and Dev texture stands for Development textures. It's there to help you develop (during blocking-out or brushwork stage) your map. It's a tool to help you build the map. Up to you if you use it or not. I'd recommend it as it allows you to clearly see miss-aligned textures, scaled textures, etc. It also helps you to build on-grid, which is so important for clean brushwork and to prevent problems later (like leaks, overlaps, etc).