It's a good idea, because it gives any followers you may have gathered since originally posting the Hub, a chance to read. And of course can 'gather' a few more pageviews for you as well...
Not something you'd want to do all the time though, because the process only reaches your followers, who'd be entitled to get ticked off if you keep pushing your stuff at them.
As a matter of interest, I just today re-posted a hub I wrote in April, and during the intervening six months, my followers have doubled, (from 'hardly any' to 'a few more', lol).
Here's how to do it:
Open up your Hub, (Not in edit mode) click on 'Share' - down by the rating buttons, you'll see a range of options, choose 'share with followers', click, and you're done.
Of course, its other (real?) purpose is to enable anyone who reads a Hub they think their followers would enjoy, to share it with them. Kind of 'spreading the Hub love...'