Data loss prevention refers to preventing unauthorized, purposeful or unintentional, access or transmission of sensitive or critical information. A comprehensive DLP solution covers data at rest, data in use and data in motion. Network DLP solutions help address the data in motion. They use MITMing in order to inspect data leaving the enterprise. They are often deployed in order to meet regulatory data protection requirements.
For example, I used to work in investment banking. It was well known, and expected, that us (and probably most other institutions) had network level monitoring, to prevent say, the leaking of a deal on some chat or web forum somewhere. Believe me, when there's lots of money involved, there are plenty of incentives to leak things.
You'd look pretty silly if you had to explain to the regulatory authorities that you took zero steps to secure your network perimeter, or prevent the exfiltration of privileged or confidential data.
And there are other legitimate use cases - educational institutions and schools come to mind.
