It's to avoid redeclaration on each loop. The each function is a bit of an anomaly because it fires off a new function for each loop but it is still nonetheless a loop (although a very slow one because of the repeated invoking of functions. The each loop is so slow that it may not make a noticeable difference.

I try to avoid each wherever possible because it is many times slower than any other looping approach and often leads to extra creation of jQuery objects (such as $this = $(this)).