Aaron Bertrand discusses synonyms:
Let’s say you have a table called
dbo.BugReports, and you need to change it to
dbo.SupportIncidents. This can be quite disruptive if you have references to the original name scattered throughout stored procedures, views, functions, and application code. Modern tools like SSDT can make a refactor relatively straightforward within the database (as long as queries aren’t constructed from user input and/or dynamic SQL), but for distributed applications, it can be a lot more complex.
A synonym can allow you to change the database now, and worry about the application later – even in phases. You just rename the table from the old name to the new name (or use
ALTER TABLE ... SWITCH and then drop the original), and then create a synonym named with the old name that “points to” the new name
I’ve used synonyms once or twice, but they’re pretty low on my list, in part because of network effects: if I create this great set of synonyms but the next guy doesn’t know about them, it makes maintenance that much harder.