Merging two contacts is typically done because you had a contact already set-up and someone mistakenly created another similarly named contact and added a job or opportunity to that contact.
What gets moved? There is a “Loser” and a “Keeper” with the merge. What gets moved to the Keeper:
The Loser’s jobs and opportunities are moved
The Loser’s Additional Contacts are moved
The Loser’s work locations
and the Loser’s jobs and opportunities are move
What does not get moved?
Loser’s billing and service address does not get moved
Loser’s phone numbers does not get moved
Loser’s email address does not get moved
The Loser’s source, tax rate, and special instructions, plus any custom field data is deleted
It is important that prior to initiating the merge that you verify the Keeper’s contact data is correct and that you don’t need to save the Loser’s contact data.