Lets see...
Say you have a hub that gives a link to one of mine as a source of more information. When the spiders come crawling your hub they will see that link and, if it's dofollow, will follow it to my hub.
I thus gain some "google juice" to my hub because you found it valuable enough to link to it - it will help raise me in the SERP's. Because my hub is so massively authoritative (don't I wish!) you might even get a little juice to your hub because you are using such great information that came from my wonderful hub. I might get indexed a little faster because the spiders found the link and, through it, found my brand new hub as well.
On the other hand, if it is a nofollow link the spiders don't follow it, don't see my hub and neither you nor I get any benefit from it as far as SEO goes. Readers can still click it and visit me, which is nice, but it won't help raise me in the SERP's.