Depends on what protocol the hub uses to communicate to the devices. ZWave and Zigbee are standards that are often compatible with generic hubs and Home Assistant. Gotta do your research on each device and how well they are supported

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

Most smart home devices are actually not that smart. They are simply a switch with wifi, which logs in to your local wifi and shouts “HERE I AM, I HAVE TWO MODES, YOU CANSWITCH BETWEEN THOSE” into the ether.

Reasons to have a hub are:

1. Not having to open the website of every single device or send a command from the command line if you want to use it, but rather have one place which registerss all devices and bundles them up to serve you in a nice web interface or app.

2. Have more advanced/intercinnected functions (not only: send “up” to the blinds device but send “up” to the blinds device, if the time device says its later then 8:00 and the daylight sensor device senses dalight.

3. Some hubs have security features, like claiming the device and establishing a password, so not everyone in you WiFi can do everything.

4. Good ones act like a protective layer from you to the outside internet (esps and most smart home devices are simple devices, which are rather unsecure and where you don’t know which code is running (your Chinese security camera might not only send pictures to you but also to China, and you would not knowbif its connected to the internet directly). So you can only allow connections to the hub, and if you have an open source hub like HomeAssistant, you could be rather certain stuff like this will not happen as easily.

5. Do all the heavy lifting with more CPU power (user and password login, updates, scripts, voice recognition if you want stuff like this, keeping date and time recently updated, merge (in the case of HomeAssitant) different protocols (like iqtt, phillipsHUE, ZigBee, etc. Pp.) Into one coherent system.

Some other things also, and that being said: it is literally the device controlling your entire house and privacy, so get one you trust, preferable an open source one where you don’t have to accept agbs to give up all rights to Samsung or so.