In most old-school games, especially in the types of sandbox adventure where there's no time sensitive mission involved, wandering monsters provide the only disincentive for spending infinite amounts of time checking every wall and every item, thereby eroding some of the crucial decisions that make the game fun. Wandering monsters are, essentially, the measurement of time ticking.
Many people don't like wandering monsters because the whole concept is a bit unrealistic -- infinite numbers of monsters roaming around? But they play an important structural role in a sandbox adventure, one that needs a substitute if you eliminate the concept based on realism.
Over at The Dragon's Flagon, Waywardwayfarer has proposed a tiny little trick for making the wandering monster checks a bit easier to track. It's basically a change in probabilities so that the check can always be made each turn instead of using one check on staggered turns.
Take a look - it's simple and elegant.
Doctor Who RPG at Humble Bundle - Starting at a Buck!
49 minutes ago