People often look at things like the current Friends and Family Alpha test and wonder why they exist. Why not go straight to beta, or even just do everything via internal testing? Well, there's lots of reasons. One of them is one expressed very well in this tweet by Russ Petersen
What we've seen happen through several betas is that content has been changed, reworked and redesigned based on Alpha or Beta feedback. I'm reminded of the Cataclysm
expansion and Paragon's effort to be World First on Heroic Sinestra
, a fight that simply hadn't been tested very much externally, in order to keep it relatively unknown. This meant that it was a heroic-only boss, being attempted for world-first status, while the fight was being hotfixed. It was necessary to preserve some of the mystery of the fight, because let's face it, if it had been open to testing someone would have written up the strategy and Paragon would have been going in knowing what the fight was and wasn't. But it also meant that Paragon had to learn what the fight was and wasn't while it was being changed, during the attempts
Similarly, the drastic revamp of the Jade Forest
beta can't be undersold here. This happened right in the middle of the beta -- players who were testing out content had to be moved to the next zone and leveled up while the Forest was completely redesigned. In both cases, we see how the presence or absence of testing can have wide consequences for everyone.