Descriptions can definitely play a huge role in how your avatar is received.
On the cosplay side you can give information about the character and/or scene you're trying to do, just so people understand why you did what you did and used the items you used.
The original side it can help make the avatar come alive. When someone can see the avatar and the story working together, they're more likely to give you a better score. I hate it when I see an avatar that says "just something I threw together" or something like that, just because it doesn't seen like they actually care or took pride in their creation. Even a small description about how you came up with the avatar can help.
I definitely take the description in consideration, only if the avatar somewhat interests me. There are just some that shouldn't be in the arena at all, when I see them there is no saving it with a good description. Any regulars know which ones I'm talking about.