One of the biggest differences between classic and modern team sites is that modern team sites are connected to Office 365 Groups. Groups let you choose a set of people that you wish to collaborate with and easily set up a collection of resources for those people to share; resources like a shared Outlook inbox, shared calendar or a document library for collaborating on files. You don’t have to worry about manually assigning permissions to all those resources because adding members to the group automatically gives them the permissions they need to the tools your group provides. Office 365 resources are more flexible, and are easier to work with.