It's about clear communication of a commitment to users and licensees. It's about building trust, and hence strengthening and broadening the community around the software.
I licensor can know that they never intend to re-license code commercially, but their users and licensees can't know that in the same way. Without a clause like this have to take that on trust and some might choose not to do that, missing out and weakening the adoption of the code and it's community. The presence of a clear commitments avoids that problem.
And those derivative works do not have the option to use a less restrictive license that allows such behavior.
Not sure of this though.
But is that true? Basically, the license is copyleft now, but if certain conditions occur in the future, it will become BSD-like (let's go with that designation for familiarity).
Thus, the license is effectively (copyleft | BSD).
A project which uses regular old predatory copyleft can therefore use it now and continue to use it, whichever way it plays out.
But I would assume that derived works could not choose to use the code as BSD - it’s not “choice of copyleft or BSD” it’s “copyleft unless you try to do something shady.”
Somehow..?
Why would anyone choose this?
It must be for the sake of giving the downstream users some sort of assurance that a Bad Thing won't happen to them, as a sort of promise.
"If we ever commercialize this, we will give all of you the opportunity to do the same."
It seems that the only authors for whom it would make sense to choosing this license would be authors who have no intention of going mixed proprietary licensing now, or in the future. Moreover, authors who believe in copyleft FOSS licenses and want to keep the project that way.
Someone who chooses that license knowing they will likely issue proprietary versions might as well skip straight to a simpler BSD-style license that is implied in that action.
Someone who doesn't believe in copyleft FOSS would also just skip this sort of thing and give the users the "hyper-permissive" license.
No matter what promises a copyright holder makes, they can retract them. However, at least this will be in force for users who hold existing copies. So that is to say, if the authors decide to commercialize and switch to Affero GPL, then only the new issues of the software going forward will be bound by that AGPL; the existing users will have the "hyper-permissive" license for the code up to that point.