In some cases using WiFi makes sense, for example if you've got a mains powered device such as a camera, smart plug, or similar. However z-wave really comes into its own with remote battery powered devices - a properly configured motion sensor for example will easily last two years on a single 3.3v lithium battery. That's despite waking up every few hours to report its presence, and probably transmitting the presence of motion, temperature changes, light levels, and other data points far more frequently.