The idea is that something (some metric) will spike/dip in that period that is different than normal ("Why do does our petty cash usage fall when X goes on vaca

The idea is that something (some metric) will spike/dip in that period that is different than normal ("Why do does our petty cash usage fall when X goes on vacation?"). It also means someone else will probably handle some/all of their "caseload" (aka "work") and might notice inconsistencies ("Why are we paying $X to Odd Corp monthly?"). Bottom line people are sloppy and/or their schemes can't handle them not being constantly present or "sunlight".