To be fair, let me post a website of an organization that supports tail docking:
Tail docking - the case for tail docking
Their arguments in favor of tail docking are (1) it reduces tail injuries in the docked breeds, (2) it improves hygiene in long-haired breeds like Yorkies, and (3) it maintains breed standards.
Of course, they don't cite any research in favor of any of these conclusions (except for some anecdotal information on tail injuries in Swedish dogs following a ban on tail docking), and if you do a Google Scholar search for research articles on tail docking, the preponderance of scholarly articles oppose tail docking...