Zigbee and Z wave are both third party communication protocols used for home automation equipment. Smartthings is officially certified for both.
https://support.smartthings.com/hc/en-us/articles/208672926-Z-Wave-and-ZigBee-FAQ
Smartthings can use “Geopresence” so that the hub recognizes when the phone has come close to your house. (Not close to your door specifically) how well it works varies from house to house. See the following FAQ
That’s different than a lock which recognizes something getting very close to the lock. That feature is not available for the Kwikset model that you mentioned.
There are lots of different ways to handle presence, but most people find it’s enough to just use a “zone” which is based on your phone presence. But see the FAQ for more discussion.
As for why some people instead want a key or NFC tag instead of just the “zone” associated with the phone, it’s typically because they want a much smaller detection area, maybe only a few feet from the door lock. If you just rely on phone location, your detection zone is typically 500 feet or more. Some people prefer to wait until they are actually almost at the door before the detection triggers. 