Don't get me wrong--it's not like I want to read your messages and very likely won't. But there are times when I have no choice. A few years back, a group of interns started privately harassing other interns via Slack. Only way to see it was to boot an offending intern from his work station and go into his Slack to see what was happening. We had to make all intern accounts into multi-channel guests after that. Compare that to our email, where I can go into anyone's messages immediately if need-be. This is all very standard corporate IT stuff that you need for HR and legal reasons.
Edit: I'll say this is still not an ideal solution. I don't go into private communications unless I have to, and I'd rather have the option to review specific DMs / private channels than dump everything. I really don't want everything; that's more than I care to see. Also, to clarify, I'm in the US and our employees are well aware that communications on company-operated platforms should not be considered private. I want them to be careful how they communicate in writing, not because they should be worried about me, but because they should be worried about Slack getting hacked/leaked. With the recent Facebook news, I should have thought that sort of concern was obvious.