Why is internet necessary to sync Sense 2?

[zballa]

I hope someone can explain to me why the watch needs access to the data plan on the phone to sync to the phone when the watch already connected to the phone via bluetooth. The only explanation I have is that the watch send the data to a Google server and the phone connects to the server and download the data, which you see after the sync. I could be wrong.

Moderator Edit: Clarified subject

[phatbitzr]

The Sense 2 does not require data plan, but a data plan is required on your phone as it is required to sync the watch's data to fitbit's storage.  Technically, you don't need a phone or data plan, as you can use a wifi tablet to perform all your syncing.

The phone does not store any of your fitness data.  As you are syncing your watch, it is transferring that data to your phone, which at the same time transfers it to fitbit's storage.  It is then your watch is able to purge its synced data.

[zballa]

So, there is a Fitbit entry on my phone, I have an iPhone 14Pro. There are different settings that you can turn on or off, one of them is Cellular Data. If I turn it off the watch does not sync to the phone. Also, if the Wi-Fi is enabled on the phone the watch does not sync either. I cannot use my Wi-Fi at the same time when I try to sync the watch to the phone. When I am home my phone is on my Wi-Fi, since the whole house is on a mesh network. 

[JuanFitbit]

@phatbitzr Thank you for your help .

@zballa , Nice to see you around. Thank you for your messages. 

As our friend phatbitzr mentioned, you need an Internet connection  (either Wifi or mobile Data) to sync your Sense 2. As all your data will be saved into your account. 
The connection between your phone/tablet with your Sense 2 is over Bluetooth. 
I'm not quite sure to understand when you write " I cannot use my Wi-Fi at the same time when I try to sync the watch to the phone."  Could you please elaborate on this? 
Keep in mind that the Sense 2/ Versa 4 and Versa Lite cannot be connected to wifi. For those models that can connect to Wifi, is to download playlists and apps from the Fitbit Gallery, and for faster firmware updates.

I suggest you to have a look at these articles: How do Fitbit devices sync their data?

Why won't my Fitbit device sync?