This item arose as an alternative to sidekicks to give players more control over their NPC companions. You could easily swap out the occupant of the bag with a small sized monster that wants to accompany the party (or the party has forced to become its mascot). Because the user must use a bonus action, this item can be less powerful than sidekicks. However, it is a good option for spellcasters who lack proficiencies with weapons.
Sorovian Lore: Joff the Deep Gnome created a bag for himself and Groblin to be carried around by the party.