What is the purpose of random encounters?

[Mastikator]

Thanks for the responses. This is giving me a much better understanding on the issue.

On a related topic, what do people think about literal wandering monsters? Ones that have a concrete existence, but move around.

The actual event in game that prompted this discussion wasn't about a monster generated from a table, rather a monster that patrolled its territory and came up behind the PCs while they were exploring, blocking them off from the exit while they were already beat up and not in a good position to actually fight it.

IMO I think moving monsters make more sense than ones that sit still, unless you happen to catch them when they're eating or sleeping or something.
It is a thing that people sit around doing nothing, but I don't think it should be the default for most creature types. Undead and constructs are the exception to the rule, wandering is the default behaviour of monsters IMO.

It does suck for the group though, but I think approaching a dungeon like Diablo takes away more than otherwise.