The phrase to wit, meaning namely or that is to say, is primarily used in legal texts and speech, though it sometimes spills over into other types of writing. In general, unless you’re going for a formal tone, to wit bears replacement with one of the many alternatives, such as namely, specifically, in other words, more precisely, or to clarify.
Here’s an example of to wit used in a legal context:
The indictment charged that Broadnax “did knowingly possess, in and affecting interstate commerce, a firearm, to wit: a RG Industries, Model RG 31, .38 caliber revolver, serial number 019420.”
In spite of to wit‘s highfalutin tone, it is becoming more prevalent in news and blog writing, where it has a formal ring—for example:
One side argues that our lives are most affected by genetics, to wit, inherited traits that are passed to us by our biological parents. [Union Leader]
The reasons for last night’s failure were many and varied. To wit: Anne Hathaway and James Franco were a terrible match. [Daily Herald]
Still, people seem find to wit useful, so there’s no reason to think its spread will slow.