This should be useful in discussions with friends of friends, or if you're trying to get large groups of people you're not connected with together (to plan a party, or talk about community issues, for example). Groups can support up to 250 people at one time, though, so you might want to turn your notifications off.
It'll also be easier to add people to group chats as the update also features joinable links. Anyone in the chat can create a custom link and share it with whoever they want to join the conversation. Tapping the link puts them directly into the group, or gives them pending status until an admin approves the request, depending on your control preferences.
Facebook has been adding to its arsenal of Messenger features for some time. Over the last year it's introduced @mentions, reactions, group payments, customized chats, and real time voice and video conversations for up to 50 people. More recently it made these features more accessible to those with low-budget phones by integrating video chat into its Messenger Lite app. Now it's only got to shoehorn some news and adverts in there and it'll be a network all of its own.