We use this domain to render user-provided code snippets. Some of our websites have enabled interactive code snippets which can be run by other users in order to see the output. This domain is used to host the results of running those snippets. We host the results on a different domain because we must prevent same-origin from being in effect. If same-origin was possible, users could write code that caused other users to do things that they didn't want to on their Stack Exchange account. What kinds of things? Evil things. Things like logging a user out, or updating their profile, or even possibly posting questions or answers! So, this domain stops bad people from doing bad things.