With "Location Assisted Sync," your contacts will be automatically synced whenever you arrive or depart from a location. This allows the app to sync your contacts more frequently. This setting is not enabled by default and it is completely optional.
"Location Assisted Sync" can be found by going to Contacts Sync > Sync, and selecting "Auto." The setting will only appear if you have auto sync enabled. See screenshot below.
When you first enable this setting, the app will request "Allows Allow" access to your location, so that it can be notified in the background whenever you visit a new location.
The app uses Apple's Visits Location Service (Visit Monitoring) in order to detect your location. It uses minimal battery power.
Your location is not used or stored by the app. Changes in your location are strictly used to trigger auto syncs.
Other well known apps that also use your location to trigger work in the background include:
- Dropbox - Uses location to trigger background uploading of photos (https://help.dropbox.com/mobile/location-data)
- IFTTT - Uses changes in your device's location to run Applets more regularly (https://help.ifttt.com/hc/en-us/articles/115010193327-About-iOS-background-syncing)
- OneDrive - Uses location to trigger background uploading of photos (https://support.office.com/en-us/article/troubleshoot-onedrive-for-ios-app-problems-15d3efc1-1b5e-4d3a-90aa-b6c48ea6bf71)