These bolt seat cups are made from alloy so thay are very light weight. Their main function is to prevent wear and tare on bolt heads but they are also used to dress up the frame. Suitable for M2 Pan head and Block head bolts.

Personally I use these alloy bolt seat cups on all of my freestyle frames where I am simply landing on concrete, and I leave them off long range frames where weight is a major consideration.
However, if you happen to strip a hex recess on a bolt, and you are using bolt seat cups, you will no longer have the option of gripping the bolt head with pliers.

One of my freestyle frames