The primary purpose of this is to enforce security for connections. Not every large organization employs content filtering, and those that do mostly only enforce a minimal standard of what is acceptable for work.
Not all subnets have Internet access, HTTPS is proxied for those that do, email attachments are scanned, and there is a large list of web sites to which access is blocked, likely including social networks like twitter and facebook. Depending on your employer, they may ask you to cease posting on social media when hired since they do not want to deal with employees causing controversy.
