Those are very similar to the old HK Match pistol weights. Other than weight, they do nothing. They allow for the use of a longer barrel, as the Match did, and if made out of steel like the Match, they would add 3/4 a pound to the pistol, a full 12 ounces. However, these are made out of aluminum though and only add 4 ounces, so I don't think they will work anywhere near as well as the HK steel match weights did.
When the Match was out, other than for bullseye match shooting, they were totally worthless, hence their demise. That is until TR came out then the Tomb Raider wanna be types could not get enough of them and drove the prices crazy high. TR was the only thing that kept the Match pistol in the public eye for as long as it was, even though HK DCed the Match about a year before TR 1 came out. The same people who would not give the Match a second look suddenly couldn't live without them and for a while, HK and others, imported the weights and supplied longer barrels for those building their own 'Match' pistols. Then the HK weights and barrels dried up. These weights fill the visual void that HK left when they quit importing the Match weights or they simply ran out, not sure which it was.
If one is into bullseye match shooting, I doubt 4 ounces is enough to do anything to keep you on target, better than nothing I guess but nothing like the HK steel match weights, and for a range gun, they look darned nice, but for a carry gun or a run and gun competition pistol, they are nothing but useless weight that will slow you down.