Protects the inside of your marker and reduces noise slightly. A cheap upgrade.
I remember the day I went out after buying the Rocket Cock (as it's referred to by some) and played with a few friends of mine. Well, through the course of the day my marker took a close hit to where my Rocket Cock was protecting the cock slot. Paint spattered all over the Rocket Cock, but the insides of my gun were safe. It saved the rest of my day. Instead of spending the next half hour meticulously cleaning out the insides of my marker, I just got ready for the next game. For 15-20 dollars, this upgrade is something you simply CANNOT live without.
Also, you don't have to worry about your marker's cocking bolt catching against anything while it's firing, the Rocket Cock gives your Custom no moving parts on the outside. It's constructed of high impact plastic and you should have no worries about it crapping out on you.
Buy it because it's cheap, durable, and covers that one chink in the 98 Custom's armor, the cock slot.