The fact that there was invisibilised third party access to the accounts used as the basis for prosecutions is important in and of itself. But I'm not seeing much about the underlying reasons for it.
Fujitsu knew that Horizon didn't work properly before it was rolled out to the Post Office. They were told by their own engineers that parts of it had to be rewitten because they were so shoddy. They chose, instead, to have a team of people correcting errors in the background, without disclosing this to subpostmasters or, apparently, the Post Office.
The concern is not that Fujitsu's trouble-shooters might be deliberately falsifying accounts, there is no obvious motive for them to do so. But it does make it clear that the ramshackle system did not work properly, that Fujitsu knew that it did not work properly, and that the only errors which could be corrected were the ones that got picked up centrally, with the process for correcting them creating the potential for more human error.
Fujitsu bosses knew about Post Office Horizon IT flaws, says insider
There's an interesting report on the Justice for Subpostmasters Alliance site also: Origins of a disaster (and long form version).
It is well-documented that the Post Office’s Legacy Horizon was a reconfigured version of a disastrously flawed parent project, the Benefits Payment Card. The impression given by three Secretaries of State to a Parliamentary Select Committee in July 1999 was that, once the BPC was thought to be irredeemably faulty by autumn 1998, all efforts were then focused on the reconfiguration into the Horizon project as we know it. But their evidence was far from complete. In late 1998 the Prime Minister, Tony Blair, who had been warned of the system’s instability, was asked to decide the future of Horizon. The No.10 Policy Unit had advised on cancelling the BPC and the Law Officers had given a clear view on how the public sector might terminate the project. Blair’s steer, however, paid no heed.
Many extremely well-paid heads need to roll.