The weights are for full auto use to help prevent bolt bounce (like the "rate reducer" on AKs). It's mostly to prevent out of battery discharges on some weapons or jams triggered by anti-out of battery discharge devices in full auto. It's deemed often to be unnecessary on semi-auto weapons (I don't know why) or maybe to get around NFA/ATF stuff.
In Europe, the civilian clones of the FB Grot also don't use full auto bolt carriers with the weights not installed. Though oddly also in Europe, the HK243 (civilian G36 clone made by HK), though I'm not sure if it uses a full-auto rated bolt carrier (I believe that the G36 also uses weights in the bolt carrier to reduce/prevent bolt bounce), it uses an automatic firing pin safety missing from mil-spec G36s (though HK416s/MR223s, HK417s/MR308s, I think maybe HK MR556s and MR762s, and for sure HK433s and 437s use the same type of automatic firing pin safety).