Generally dev textures are used to roughly block out a map so it can be tested quickly, using the dev textures also lets testers know they're giving feedback on gameplay, not aesthetics. The dev textured blocks can also be swapped out easily if they don't give good gameplay, instead of spending hours detailing something and finding it doesn't work and having to delete it all, by using dev textures and blocky shapes you're only loosing minutes instead of hours or days work.
The default dev textures of pure grey/orange can be quite confusing in team bases, especially symmetrical maps, so I whipped up some red/blue variants so that testers can quickly tell what side of the map they're on. Their popularity rose and eventually when hoodoo went official I asked valve to include the dev textures too, which they did.