Like heavy bottom caps, the additional weight on the mouthpiece is supposed to add to the famed slot-lock. Although it may be very real to the player, computer measurements of impedence, Q and other James Bond stuff find no diffference. My experience with added weight on the mouthpiece involved a set of custom Curry mouthpieces. Adding a Monster Sleeve did make the sound darker by about 1 Bach cup letter (to me as a player and also to colleagues listening to the with/without combinations) while retaining the upper-register comfort of a shallower cup. A player who sounds just fine on their normal weight mouthpiece and is happy has no reason to switch to or even try a heavier weight. They do do something, for some players positive, but I would warn against getting into the "how many grams is ideal?" kind of safari. That would lead to madness.