It is the corporate internet. The internet exists in the box, as far as your employees / security model are concerned. (as far as the security model is concerned, There Are Always Bugs - this is a feature).

If it weren't for the need for remote backups, email and such would be hosted there as well, and you could run a company on one of these with no access to the public internet at all. Accounting, finance, etc: all of it.