Long-term documentation is not the reason logs are useful. Asynchronous communication is the reason logs are useful.
That is, you can ping people who are offline, or with a flaky network connection (e.g. mobile, where you can't keep a consistent TCP stream open), and they can still see your message and a bit of context when they get back.
Long term documentation should be not be in any emails or IRC logs. It should be properly documented as a formal documentation. But that is different to coordinating a team.
That is, you can ping people who are offline, or with a flaky network connection (e.g. mobile, where you can't keep a consistent TCP stream open), and they can still see your message and a bit of context when they get back.