I thought so too first but then I looked it up on the link that Krywiggles provided. The reason they had the jacked is because when you have the jacket on it creates such big and heavy shrapnel that it is too dangerous to use in the offensive role. It was rather common during ww1 and ww2 to use high explosive hand grenades during assaults because they didn't give off as much shrapnel and therefore it minimised the risk of friendly fire due to shrapnel. Often fragmentation handgrenade were employed as a defensive grenade for when you are defending a fighting position or a trench the reasoning was that most of your own infantry will be protected by their fighting positions and only the enemy will be exposed to the massive hail of shrapnel.

The concept of offensive and defensive handgrenades fell out of favour because eventhough they are safer during an assault high explosive hand grenades simply aren't very effective unless you throw it into an enclosed space where the pressure wave will incapacitate the enemy. So today most handgrenades are fragmentation grenades.

Also the Germans developed a similar sleeve for use with their M24 and M43 grenades they called it Splitterring. Picture at the bottom of this page.