There are times when you buy a mouthpiece or have owned a mouthpiece for a number of years,were they don't play the way they should or might have nicks to the tip and rails or the table might not be flat and then you can send it away to have it refaced and perfected and they usually come back playing better.
There are also other modifications that can be made like adding a baffle to a non baffle piece or like sending a standard Otto Link STM mpc to Morgan Fry and he will turn it into a florida like mouthpiece by adding material to the baffle and at a fraction of the cost a real florida would cost.
Saying this the best 2 mouthpieces i am using on alto are factory originals but i have a sopranino mouthpiece that was made better by Morgan and have used Lawrie Waldron on a number of pieces in the past.
Refacing means flattening the table, adjusting/correcting the curves on the rails and adjusting the tip. Also increasing/closing the tip opening. The work's all manual and skilled. As said above the term is loosely used and may also include internal adjustments like baffle/chamber/rail/sidewall adjustments.
Reasons for doing it:
- fixing manufacturing faults,Mouthpieces aren't always made that well, tables are often curved/warped. Rails are often unevenly finished, or have different curves on each side and not properly aligned to the table. Same for the tip. Makes them difficult or impossible to play.
- compensating for wear, mouthpieces wear and over time don't play as well or the same as they did when new.
- changing the playing characteristics (longer/shorter curve, different curve, different tip opening).
- Often done as part of a tweaking process to change the sound of the piece.
Hand made/finished mouthpieces from the good makers e.g. Morgan Fry, Ed Pillinger (but there are many more) don't need faults correcting, but you may want to change the way they play.
If you want to know more/see how it's done, there are some good ones on youtube by mojobari, aka Matt Stohrer, a top US refacer.
A lot of players use ancient gear. My alto mouthpiece is at least 60 years old. It's silver plated brass and is looking tired but still works fine. My Tenor is ebonite and even older. I looks like it has been refaced at some time. Simply playing it for 80 years or so has produced wear that needed correcting. The old S80 I use on baritone is modern plastic and shows no sign of wear after me blowing it for 25+ years and I doubt it will ever need to be refaced.
Mouthpieces are like shoes. If they don't fit you can't play. In this age of ebay and trans continental communication there is a wealth of gear available. My tenor mouthpiece came from Spain. Back in the day when I started out the best resource for gear was junk shops and so I think a lot of making what you had into what you wanted was more prevalent.
Looking for that perfect sound is why good and bad gear gets experimented on. I expect that sometimes it works and sometimes it doesn't, and the search goes on.