vacuum

Was pulling my carb for a rebuild and wanted to ask you all for an explanation on what purpose this vacuum serves mounted underneath the distributor with a line going over to the front of the carb. Not sure what it is or what it does.

Hi I'm looking for the cable and dash assembly, I have the nut and the motor but need everything else. If anyone has replaced with an electrical swop out perhaps you still have the parts I need? thanks Garth