As the Cast plan a move against the Big Bad something else comes up, often a reminder of how the Cast have to straddle the normal world as well as the weird one.
Grounded on your way to fight the monster? Stopped chasing a monster by the Principal? SATs?
Moved along by police while staking out a lair? The vitally important artefact has been sold at auction?
Or similarly, the Cast could be attacked by a force of good (or good-ish) like the Initiative or the Knights of Byzantium, because they're supernatural and therefore suspect.
Or, horror of horrors, you could be surveyed by the Watchers.
A slightly demented example from the start of TWH season seven - the typical "archaeological dig unearths ancient evil" plot was complicated by the film crew from Time Team.
As well as a reminder that the Cast generally have to operate in the "real" world and maintain secret identities and the like, it widens the setting.
Mundane(ish) inconveniences can lead to recurring characters, often thorns in the Cast's sides that they can't do anything about directly.
Meeting other good guys - possibly including the classic misunderstanding leading to a fight between superheroes - can make the fight less lonely than being the only heroes in the entire series. And it gives you someone clued-in to kill off to show how scary the Big Bad is.
Example: Wrong Direction
The band's van (by which the band mean Zora's van) is impounded. With the Stone of Frost, the only thing that can neutralise the Stone of Fire, inside.