Typically for devices that require a hub, it’s because they do not have WiFi chips. Often manufacturers leave off WiFi chips because the power draw is too high for the batteries, but there are other reasons too.
The hub act as a bridge between whatever local networking protocol is used (zigbee, z-wave, BLE) and the LAN. For example, a lightbulb may use z wave to talk to a hub, the hub uses Ethernet/wifi to connect to the LAN, the LAN connects to the WAN.
In some cases, you can setup a system that controls everything locally without needing an internet connection at all. This is often preferred in the home automation space by users who want to ensure reliable access to devices.
Some centralized hubs can talk directly to many brands of devices without needing many proprietary hubs, but they tend to be expensive or require a licensed dealer to install.
For a DIY option, look into Home Assistant running on a raspberry pi
I use
Arctic for Lemmyon iOS