Resetting Sync database will delete the database completely with all the values in it.
I think one should reset the sync database as the last thing when all troubleshooting tricks provide no result. Only consider resetting when:
- Your import user profile connection is fetching details from AD but they cannot be seen on SharePoint
- Similar scenario with export connections, so you want to push changes back to AD but you don’t see any changes on AD
- You have modified settings on your FIM Client manually
Here is a very nice article on resetting the sync database you should follow before considering a reset:
PS: The User related information such as Profile Pictures, About Me etc. resides in Profile DB and the Social DB, these information will be safe as you do a reset of your Sync database