The Annotated Order of the Stick - A Companion Guide

[illyahr]

I thought the cross-hatch pattern was to make the case break apart into shrapnel.

This. The cross-hatch pattern makes the grenade shell break apart along designated lines for an even shrapnel spread. Otherwise, you might end up with a large chunk flying in one direction with small shards flying in the opposite direction.

Modern grenades do this too, the cross-hatching is just on the inside so it isn't visible.