Why /usr/bin/env in scripts

I notice that the first line of the pasky shell scripts is

#!/usr/bin/env bash

rather than what I'm more familiar with:

#!/bin/bash

I see that env can be used to alter the environment before running a program, but that doesn't seem to be happening here.

In fact, on my system /usr/bin/env is a link to /bin/env...

So what am I missing?  Is this a portability aid?

Thanks, :v)