One of my co-workers is leaving our company and had an info session on things that he's been doing so that others can continue the work after him. I won't be doing his stuff, but there were all kinds of little things that will probably be useful and also the overall picture got again little more clear. Many things were not essential for my work or have no immediate use, but knowing more about the system and people helps when you consider what and how it should be tested (and developed).
It might be good to have this kind of slightly formalised information sharing more often also from people who are not leaving. If everyone in the team would give this kind of brief once a year, others would be more up-to-date and could for example more easily act as replacement during holidays. Wide, but voluntarily coming audience would probably work best so that people don't feel they are wasting time by listening things they already know or that are not useful for them.