User blog comment:Terrari492/Obsidian Generator/@comment-4994353-20120530182338/@comment-5014408-20120530213456

It's for two main reasons:

1) I use obsidian bricks and walls a lot in my house/structures.

2) Obsidian skulls sell for 54 silver each, and that's only 20 obsidian.  Money is nice.