If you are just talking about why hiring Caserio might be good news he has been a sought after candidate for GM for a number of years by a number of teams. He is also pretty much the only person that BB and the Pats have ever fought to keep, their stance has always been if someone wants to leave let them leave and he is the ONLY person, other than the Krafts, listed on the Pats website as being part of the F/O. None of this of course means that he would be a good GM but then there really are no guarantees and if you are going to hire a first time GM then he's got about as good a pedigree as you can ask for.
If you mean why was firing Olsen good news I can't say it was but then no one else has said its good news just at most that it might be a sign of things to come. What I don't get is this believe that OB fired him to "get rid of the competition" when their jobs are completely separate and had little to do with each other. Also not sure where people keep getting this idea that OB wants all these jobs other than their own dislike of him and letting that color everything he says or does. Really I've never seen or heard him say anything to indicate he does want all these roles. That being said I haven't seen him give any of them up yet either but the off season is just starting.