Because I would very much like it if we could post multiple threads in the art board for single pieces of art (or single threads for a series or drawings.)
I know a lot of people generally setup single gallery threads, but most forums I go to now just have an open rule system for their art boards, where you CAN make an all-encompassing gallery if you want, or you could make threads for a specific series of drawings (like if you have a specific kind of art you want to work on at that time), or you could just make individual threads for each piece if you feel it warrents it.
I understand why people like having galleries, but I actually find that people are more likely to comment on a piece on a forum if it has it's own thread. If they want to see a gallery with all my stuff, there's always DeviantArt. On forums I prefer to get specific comments on individual pieces, so that's my preferred method of posting them.
So is it against the rules to do that, or is this art board open to different formats of posting art?