On the N&W, it was to provide shelter for the Brakeman. To my knowledge, there was never any hesitation about pushing on a caboose.
As for what's inside; a seat, probably a hand tool or two, and maybe a lunch pail. They were used on all N&W "A's" and "Y's". At the very end of the steam era, the J's (even the 611; I have a picture) had a dog house.