What does an ensemble cast bring to a movie?

In the wake of Tinker, Tailor and with Contagion and the terrifyingly star-filled New Year's Eve on the way, we ask what ensemble casts add to movies

To generalise for a moment, ensemble casts are more often seen in television shows than on the big screen. Having a number of characters who hold equal status or importance to the ongoing narrative is a format upon which 20-something-episode seasons of a show can be sustained, but isn't quite so effective in the finite time-scale of a movie.
Nevertheless, filmmakers like Quentin Tarantino, Steven Soderbergh and Edgar Wright have consistently deployed ensemble casts in their movies, with Soderbergh having notably gotten a whole Ocean's trilogy out of one big, starry ensemble.

Essentially, there are two types of ensemble movie. You can either tell a single story in which every character is played by a good or at least recognisable actor, or go down the increasingly prolific route of showing a number of interweaving narratives populated by stars given equal stature, a method of storytelling that is closer to the complexities of popular television.