The more senior people in organisations are, the more vulnerable they are in disclosure processes, because nobody has oversight of them day to day. Just look at Paula Vennels in the Post Office scandal.
In my career I have seen senior people who were highly intelligent and capable say amazingly incriminating things in emails. Given that Masters consistently appears to be anything but highly intelligent or capable, I can fully understand why we would want access to all emails from the organisation he was running, and why the PL lawyers would oppose it.