A home already has Wi-Fi, yet smart home devices keep introducing additional radio protocols. The reason is that a door sensor and a laptop have almost nothing in common as network clients.
Wi-Fi optimizes for the wrong thing
Wi-Fi was built to move large amounts of data quickly, and its association, authentication and keep-alive behavior all assume a device that stays connected.
A battery sensor needs to send a few bytes occasionally and spend the rest of its life asleep, where the cost of rejoining the network dominates its energy budget.
Low-power protocols invert those priorities, accepting low data rates in exchange for radios that can wake, transmit and sleep in milliseconds.
Mesh routing extends range without more infrastructure
Mains-powered nodes such as smart plugs and bulbs forward messages for battery devices, so coverage grows as more devices are added.
This suits a house well, since the difficult locations for radio are usually basements and garages where a powered device already exists nearby.
Wi-Fi has no equivalent by default, which is why extending it means buying access points rather than benefiting from the devices already installed.
Network scale is the practical limit
Home routers are sized for a moderate number of clients, and consumer hardware becomes unreliable well before a heavily instrumented home would finish connecting.
Moving dozens of small devices onto a separate mesh keeps them off the router entirely, with the hub appearing as a single client.
That separation also isolates them, so a compromised bulb is not sitting on the same network segment as a laptop.
Thread and Zigbee differ in how they address devices
Zigbee defines both the radio and the application layer above it, which historically produced interoperability problems between vendors implementing the upper layers differently.
Thread carries standard internet addressing over the same class of low-power radio, letting devices be addressed like any other network endpoint.
Application-layer standards then run on top of Thread, which is the arrangement current interoperability efforts are built around.
Border routers are the piece that ties it together
A low-power mesh cannot reach the wider network by itself, so a border router bridges it to the home Wi-Fi or Ethernet.
These are increasingly built into speakers, displays and streaming devices rather than sold separately, which is why a hub can appear without being labeled one.
Having more than one is normal and beneficial, since redundant border routers keep the mesh reachable when a single bridging device is unplugged or fails.