Provenance. I can doctor my local logs just about as quickly as I can type. True, it's a bit harder than some people estimate to keep the whole thing coherent, but that just means that you hear about the people who get caught, but not the ones who do it successfully. If I am capable of doctoring Slack's copy of the logs with sufficient effort, it is certainly orders of magnitude harder and much legally riskier (as I would be committing many felonies in the process).
The parent I was replying to said they ought to keep the Slack logs themselves. If they're logging into Slack to get them, then it is Slack doing the keeping and we're back to Slack doing the storage.
