In Chapter 8-Before he left, Gandalf said he believed the dwarves need luck, courate, and sense if they were going to accomplish their mission.

I suppose Bilbo was pretty lucky to kill the big spider on his own in the dark. I think they have "luck" on their side though in the form of the ring. Bilbo has the ring which, when used in moderation, can get them out of most any jam. Bilbo uses the ring to thwart the spiders' plans. Bilbo, being invisible, frustrates the spiders and lures them away from the dwarves. Bilbo then doubles back and frees the dwarves.