Almost all of Jetpack’s features make use of heavy processing power (our image CDN, related posts indexing), heavy database usage (stats), or require connections to third-party systems that can be very complicated to setup by yourself (Publicize’s automated sharing to social networks, email subscriptions, etc).
We take care of that all on WordPress.com’s server cluster so your hosting provider doesn’t suspend you for using excessive resources.
In order to do any of that, we need to sync the data outlined at https://jetpack.com/support/what-data-does-jetpack-sync/ to our service, and we’re very upfront about that during the connection process prior to anything being synced.
Further details on that page outline what we sync, and of course we only sync data required for _active_ Jetpack modules, meaning that we only sync what we need to provide the features you want.
All synced data is handled under the terms of our Privacy Policy: https://automattic.com/privacy/
With regards to sales data, this is only synced if you use the WooCommerce Services plugin, a fully separate plugin. This is detailed at https://jetpack.com/support/what-data-does-jetpack-sync/#woocommerce-services
If you don’t want us to sync data to our service, we do recommend that you not use Jetpack, as it is integral to the services we offer.
Please let us know if you have any questions!
