Imagine z-wave switch as some device in between original (physical) switch and the light (or any other device which was connected to the original switch).
If you don't use z-wave functionality at the moment, this additional device is "transparent" to the installation, which means that it just pass commands from physical switch to the light as in regular installation.
The benefit is that
in addition you can manage it via Z-wave controller.
You have two tpes of in-wall switches: one is built as a physical switch which you put in a place of original one,
(see for example:
www.domotics.sg/review-mco-home-capacitive-touch-z-wave-switch/ ), the other is a device which you put into in-wall box behind original switch, and connect to the light and to the switch (see for example:
https://www.vesternet.com/resources/application-notes/apnt-111 )
What you choose depends on your needs (i.e. if you need to keep visual design same as other switches, probably the option "behind the switch" will be better than replacing original switch) or possibilities (i.e. do you have hot and neutral available at the place where original switch is installed?).
Remember that in some cases you'll need to replace original switch anyway (i.e. dimmers work better with monostable, "doorbell" type of switches, so if you want to use dimming features, you'll probably need to install such type of switch even if you use "behind the switch" type of z-wave device).