Every home is different, materials, locations of switches, etc all can make a difference. When building out your z-wave network it's always best to start with the device closest to the hub and work your way out in a "spiral". There is some great posts here in building a strong mesh.
With that said a dedicated repeater can help if you have a spread out devices. . I used one at client site to repeat a z-wave signal outside the house to a pole barn so he could turn his Christmas lights off and on.
He didn't really say that, he just said what you'll read here. What you'll read here is that you theoretically should not need one if you have enough Z-Wave repeaters in your network, which includes most powered devices. Despite this, some people have reported much better luck after including one of these in their network. (Aeon and Dome both make one and I've heard good things about both.)
I suspect this is due to the reasons stated above--and at the very least, with the Dome being USB-powered, it's a lot easier to place one of these you might want one as opposed to in a switch box or in or near an outlet. That, at the very least, might be helpful for some people.
Edited! The Dome Extender (which I do have, but am barely using--I really did need something to extend my test Z-Wave network with no switches/dimmers/plugs or anything on it) can work either way, and I got them confused.