My impression is that this was done to specifically counter Donald Trump rushing a vaccine prior to the election. I don't think it was really made to give any security beyond that
I wouldn't have thought the companies involved are primarily thinking of that. The risk to the companies is that Trump going on about skipping trials will hurt public confidence in any vaccine that is finally released, even if such skipping never happens. This announcement may serve to reassure some people that the manufacturers are going to behave themselves even if the government is negligent.
Also, probably, reassure some shareholders. The reputational risks involved in this trial-skipping stuff are horrific.
Trump has made some very worrisome statements intended on either rushing a vaccine or placing the blame for the lack of it into whoever refuses to rush it.
Then the FDA answered with a non-statement that said basically "there's no conspiracy, we are fully invested into doing our job". And, of course, that gave a clear signal that the FDA felt pressured and was reported worldwide with that bias.
So, I read the GP saying that the manufacturers are telling the FDA "we know better than you" in an attempt to conserve their own credibility. Notice that this is a very bad message to pass into their own regulator, and worrisome for different reasons. But as long as it's only PR, it's not a real problem.
This announcement was made very shortly after Trump started making claims that a vaccine could be ready "late October/early November". Trump is a known fool, and even were he not, his losing prospects at re-election, combined with the criticism of his administration's response to the pandemic make it seem like he could be pushing vaccine makers to rush a treatment to pump up voter support.
I believe vaccine makers are diplomatically responding to the Trump headline by saying "We aren't going to rush this for the wrong reasons."
I'd rather more people die to Covid than find out 20 million people have failing livers and 5 million were born with birth defects because they rushed a vaccine without enough testing.
The bigger issue is if they rush a drug that does long term harm to people, it could destroy the company. This is a drug that they will be giving out to 300+ million people in the US along. Screwing it up would result in a massive amounts of lawsuits that they may not recover from. They don't want to risk that just for some political capital with a person who may not be in power in a few months.
I feel like ending the pandemic is not a motivation that is likely to make a researcher (an expert) rush things and end up hurting more people. Greed or glory is much more likely to be a motivator for that kind of mistake.
Also, probably, reassure some shareholders. The reputational risks involved in this trial-skipping stuff are horrific.