Dead blow hammers don't recoil, i.e., bounce back, at least significantly when you strike something. In confined areas this helps prevent damage that can be caused by a hammer recoiling backward off the item you were striking, and impacting something that would be difficult or extremely expensive to replace or repair. Dead blow hammers also prevent the recoil that can cause repetitive stress injuries to a persons wrist or arm. I also heard of an incident were a sledge hammer that was being used to break something down recoiled backward into the users head, nocking them unconscious. For safety reasons a dead blow sledge hammer could have prevented this or at least mitigated the injuries. Brass, Bronze, Copper, as well as Lead and Aluminum hammers/mallets are sometimes classed as non-sparking for use in flammable or explosive atmospheres. They are also used to prevent marring on some materials. Another purpose is they have different densities and recoils so they are useful for precision adjusting of parts or machine setups. The denser materials, Brass, Bronze, Copper, and Lead, require gentler taps to adjust a setup as a lighter material would and therefore offer more control than a lighter hammer would. A number of these materials also have less recoil than steel. Plastic and Rubber hammers/mallets are used for Similar purposes as the softer metal hammers but using a material that's sometimes even softer. Plastic faces are sometimes used on heavier hammers to make a denser hammer that is both either non marring or has less recoil.
