Groups allow you share content between users in specific ArcGIS Online and ArcGIS Enterprise organizations. They also allow you share content between organizations in ArcGIS Online. The key here is ArcGIS Online - you cannot share ArcGIS Online content with ArcGIS Enterprise instances through groups. This is part of the reason why collaborations were added as they allow you share content between ArcGIS Online and ArcGIS Enterprise (and also between ArcGIS Enterprise deployments).
A secondary factor here is where the content lives. With groups all content has a single owner. Collaborations allow you to duplicate the content so it lives in two places and syncs periodically. This is important where ArcGIS Enterprise is behind the firewall and not accessible externally to an organizations network.
So in many cases groups fulfill business needs, and they don't require much configuration. Where groups don't fulfill requirements collaborations are available. You can find more information in the following blog posts: