When they announced that S Marviel Underwood was going to be released, Mike McCarthy said it was to give Underwood a better chance to catch onto another team. That's still true, he does have a better chance to find a home now than he would after the final cuts. But it appears GM Ted Thompson had another motive for Underwood's early release, he wanted to add another fullback to the roster.
There's not much to say about Erryn Cobb. He played tight end in college at Northwestern and apparently didn't play much, although those Yahoo college stats are often incomplete. Still there are only four healthy backs in camp, and only FB Korey Hall is playing at fullback. He's looked good, but it doesn't hurt to add some more depth and competition for Hall, but it sucks that it comes at the loss of Underwood who looked like a very promising player in 2005, back before his knee injury.
Update: Grant 76 makes a good point and I agree with him in the comments.