Warnings are for where you expect someplace else to know/log if it really is an error but it might also be normal. You might log why a file io operation failed: if the caller recovers somehow it isn't an errer, but if they can't they log an error and when investigating the warning gives the detail you need to figure it out.
Who proactively investigates warnings?
statistacs are someimes run and the most common investigated (normally shut up the noise)
mostly though when you are on a known problem warnings should be a useful filter to find where in the logs the problem might have started, then you use that timestamp to find info logs in the same area