Add a Grams offer a unique solution to darters who feel they would like just a touch more weight or perhaps even a slightly longer barrel.
By virtue of simply screwing a plug into your existing barrel these add a gram in weight and 5MM in length.
It's also worth noting that adding weight and length to a darts barrel can change the weep or the pith of the dart so these can also sometimes be a solution for a player who is not happy with the angle his darts strike the board at.