Everyone has at least a two-week period every year when they are completely cut off from the job. If you have been doing something off-the-books, that's enough time for it to explode. Or, if the auditing department suspects you of something -- or even if they don't -- this is the natural time to bring in forensics to dig through your books and figure out exactly you've been doing.
It's not as important in non-finance, but it can still have some use. I know of at least two cases where a sysadmin was on vacation and then outsiders were brought in to find all the backdoors he had installed.
That's pretty much it. It's a compliance-type regulation. The idea is that if you were, say, hiding a $3bn loss in an obscure book or something then it is more likely to be discovered if you were out of the office and someone else is trading/covering your books.
There have been many cases of people in finance being able to cover up significant losses of company funds (through bad trades or what not) or a long period of time simply by understanding the front/back office systems.
If you force people to leave (and cut off remote access) they are unable to maintain the pretenses required to cover up these losses, and can help drive exposure of foul play. At a minimum you'd need two people 100% in on the cover-up which reduces the chance of it happening.
I'd love to hear more about that.