Put the title "partner" on a lawyer and their billable value goes up. It has nothing to do with skill or accomplishment, and everything to do with perception.
Consumers of legal services, from the least sophisticated personal bankruptcy client to the most savvy corporate mega-client perceive more value in working with a partner.
Perhaps is the perception that a "partner" in a law firm has more of an incentive to do a good job to make the firm look good. Perhaps it is the perception that that individual earned the partnership and therefore must be a better attorney than an associate who hasn't earned it.
Many firms now have at least two tiers of "partnerships." The first rung on the latter above "associate" is typically a non-equity position. The non-equity partner can usually be fired at will just like an associate. It is usually a carrot that has little benefit other than for the ego. Sometimes junior partners are even compensated less than high level associates, or take on more risk for little more reward.
Then there are the senior or equity partners, or shareholders. There may be several levels to these partnerships as well. The top dogs in the big firms make huge amounts of money (count in the millions). The lower level ones also make substantial money. They have arrived and truly have a stake in the operation.
There are also smaller firms where the named partner or partners are little more than figureheads and the associates do all of the work. One DUI law firm (which will remain nameless) comes to mind. The owner sells the clients. That is all she does. Her associates work the cases. Sometimes clients complain that they thought they hired the partner, so why do the associates keep showing up for court? The reality is that the client is better off with the associate in this case because the owner hasn't done anything meaningful in a courtroom in years.
I think it was Shakespeare who asked "what's in a name?" When the name, or title, is partner, the answer may be "not much."