The issue here is, by installing a root certificate, you're telling your device that whoever holds the certificate's private key, and is capable of issuing further certificates, is worthy of trust in matters of whether any SSL-encrypted service is genuine. If you're sure you hold the only copy of the private key, presence of such a certificate does not immediately give other people any additional access. (However, stealing the private key from you may be cheaper for a sufficiently determined adversary than stealing one from a professional certification agency.) However, if you're installing a root certificate that comes from somebody else, such as an employer, you're giving them the ability to issue false certificates that your device will consider true, and that's where the risk of MitM and related monitoring comes from.
Note that the certificates apply to all SSL-encrypted services, not just websites. In particular, I would point out that SSL is nowadays widely used for email, including IMAPS/POPS access to remote mailboxes.