As you probably know, zwave repeats only for Z wave and Zigbee repeats only for zigbee.
Most Mains powered devices will repeat for their own protocol. The exception is usually warning devices like smoke sensors where you don’t want the smoke detector’s own message to be delayed because it’s repeating a message for a light switch somewhere. But otherwise, if it’s running on mains power, it probably repeats for its own protocol.
Zwave
Single purpose zwave extenders were useful back during the third generation of z wave when not every device used full power.
But we are now in the fifth generation (Z wave plus) And pretty much every mains powered device now transmits at full power, so you just don’t need the single purpose extenders anymore. People commonly use plug in pocket sockets, Light switches, zwave lightbulbs, And in wall micros as repeaters. They’re all good. 
If you still have older leftover single purpose extenders, you can certainly use them to strengthen the mesh, but there’s not much reason to go out and buy new ones these days.
Zigbee
Zigbee is a somewhat different situation. The zigbee profiles used for Home Automation are very low power and typically have a range of only about 15 m per device. However, you do see people occasionally add a much higher powered zigbee pro device if they need to get signal from one building to another or for use in an outdoor sensornet, typically on a farm. You can get range of a kilometer or even more from these, but they use a lot more power in operations.
But for ordinary everyday indoor use, it’s the same situation as z wave. You just use almost any mains powered device and it will also act as a repeater. Plug-in pocket sockets, Light switches, and in wall micros.
Zigbee light bulbs are much trickier than Z wave light bulbs, though, and there is a lot of variation from brand to brand. See the FAQ:
Conclusion
You probably don’t need a special purpose extender unless you are trying to get zigbee signal from one building to another. Just lay out the backbone of your network in a sensible way and your other mains powered devices can easily handle the repeating requirements for your network.
Start with post 11 in the following FAQ, then go up to the top of that thread and read the whole thing.