The Item Group Comparison insight panel is designed to allow you to easily compare several custom groups of items for the most flexibility in analyzing item sales performance between groups not easily represented by the category architecture. While the panel is labeled and organized as a "Sales" insight - it also has the ability to show you inventory metrics as well through selection of the appropriate metric in the gear selection.
One popular use case with this insight involves creating several groups of items with differing normal retail price points and using each in this insight to gauge the difference in sales performance. Because price point is not often an attribute that can or should be represented in the category architecture, it lends itself perfectly to custom item groupings. This insight panel allows you to better access this capability.