As with any contributions you do on hubpages, it increases the chances of other people reading your hubs, following you, etc. You get to post a link to your hub in the hubmob thread on the forums, and I don't think you are normally allowed to post links to your hubs in the forums.
I participated in this week's hubmob topic 'The Uncommon History of Common Things'. I read and commented on several of the hubs in this week's hubmob. I then posted mine (a history of the umbrella) and had quite a few more people reading than I would normally get for a new post. I got some good comments (including on other hubs people had gone to after reading my hubmob one) and picked up some new followers, and a bit of fan mail! I also found a couple of people to follow.
So it is definitely worth doing, I'll definitely be participating in the next one too.