You can use your regular PC if you want, but having an always-on server (the pi) makes it more convenient to use from, say, your phone.

Zigbee is a popular wireless communication protocol used by iot devices. Without the dongle you won’t have any way to talk to them.

If it’s cloud based, buy something else that isn’t.

Your Zigbee light switches won’t do anything unless the machine running Home Assistant is on. Being able to control your lights while the computer isn’t running is really convenient.