It releases memory which is being used by your script. See http://ie2.php.net/memory_get_usage.
The benefit is with scripts which are processing large amounts of data you can run into out of memory errors, see the memory_limit ini setting for more on this.
So, yes, there may be benefit, but unless you are working with large amounts of data you shouldn't need to use it.
You may also want to unset variable to prevent their value being used later on, but if that's the case it could be argued that your code needs to be written differently to prevent such things happening.
