A modern version of a pre-19th century expression: "Sir, you have come over all unnecessary!"
The above phrase meant that a male was either becoming flirtatious, or causing a female to become very attracted to them.
An 'Unnecessary Man' is a male who can be associated with this era by looking dashing or wearing an appropriate costume, causing women to be attracted to them. It is usually applied to characters in books or films, and the look is often twinned with a bad or aggressive attitude.
"I went to a Victorian fancy dress party last weekend - the place was full of Unnecessary Men!"
"I saw Harry Potter again. That Snape is definately an Unnecessary Man."