Mini vMac emulates a Macintosh Plus, one of the earliest of Macintosh computers, sold between 1986 and 1990. It can run old Macintosh software that otherwise couldn't be used on recent machines, and so helps to preserve history, from the era when the modern user interface for computers was being defined.
Mini vMac is a spin off of the program vMac. It was originally intended to be of limited interest, a simpler version to serve as a programmers introduction to vMac. But vMac hasn't been updated in some years, and doesn't have a number of fixes included in Mini vMac, so Mini vMac may now be more generally useful.