Locks the carriage.
Used extensively when doing so-called "facing" work, in which case you do not want the carriage to move longitudinally along the ways.
Other than threading, a lathe is more properly called a TFB machine ... turning, facing, boring.
If a lead screw is present, as it most often is, then the tool us universal, in that it can do TFB and thread chasing work.
There are good reasons for owning a TFB machine, especially where high production, and cost of equipment are important factors.
For a super-precision shop, one would usually have a so-called "toolroom" lathe, which is universal, and also a so-called "manufacturing"/TFB lathe.
