The most current recommendation, which came in with Z wave generation four (we are now in generation five, zwave plus), is don’t use these at all. You don’t need them. Most mains powered Z wave devices will work as a repeater and “extend“ your network in exactly the same way as the single purpose devices. That includes light switches, plug in pocket sockets, plug-in sensors, in wall micros, etc. The repeating will happen automatically. So you might as well choose a device which fulfills another purpose as well, It’s just a better use of your money.
Back in generation three, not all devices transmitted at the same level, and it was sometimes helpful to have a single purpose device that would transmit at maximum power. But these days all the devices transmit at the same power anyway, you don’t get anything from a single purpose “extender” that you don’t get from any other mains powered device.
You can read more in the following wiki article: