From the Mach4 Operation manual,
https://www.machsupport.com/wp-content/uploads/2014/05/Mach4%20Operation%20Manual.pdf5.1 Limit Switches
Limit switches are physical switches that mark the end of a machine’s range of motion. Sometimes one or more of these will double as a home switch. The hard limit switches are basically the last line of defense before major damage occurs. Without these switches the machine could slam into a hard stop at the end of travel or even send the table right off the end of the rails, depending on the configuration. Running a machine without limit switches is risking damage to ball screws and nuts, bearings, castings, people, floors and walls.
Z++ and Z-- do not do the same thing. They should be used with two different switches. There are three switches designated for each axis, two limits and home.
The limit switch at the positive end of travel connects to the ++switch input, the limit switch at the negative end of travel connects to the --switch input. You seem to have configured both inputs to the same switch. The home switch is used to establish a known position on the axis. As said above, one of the limit switches can do double duty and serve as a home switch too.
If you have one switch at the end of travel it can be both a limit and home switch. Soft limits would then be used to sense the other end of travel.
Alternately your single switch could be the home switch and it would be mounted anywhere within the range of safe travel. Then soft limits would be used instead of physical limit switches.