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Philips Hue will force users to upload their data to Hue cloud
(www.home-assistant.io)
This is a most excellent place for technology news and articles.
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.