When you create a Office 365 group, among many other Office 365 apps, a SharePoint team site connected to that group is created. Other connected apps are, Notebook, Planner etc. This Office 365 group can be from Yammer or SharePoint or from Outlook. Here SharePoint is used as a document storage for your group.
You can manage your files shared with the group from either SharePoint or from Outlook, which has a new files interface with additional options. Now, if you go the concerned SharePoint site then you should see a link on the left hand navigation menu called 'Conversations', clicking it would land you to your Outlook group page. This is because, Outlook is here used as a conversations tool using your group mailbox. All conversations are basically emails. This functionality would change if you create the Office 365 group from Yammer. In that case, Yammer will act as the conversation medium and it also have a files collaboration option which uses SharePoint document library similar to Outlook. If you use Teams, there also you will see similar usage of SharePoint document library.
So, this is not necessarily moving out from Outlook and use SharePoint, Outlook is used for a different purpose and SharePoint is just an aid to that purpose. You can still use the SharePoint Team site connected to that O365 group, but as you can understand, to make it popular you need highlight the capabilities of SharePoint online to your users. If you think of document management, then there are many other document management capabilities of SharePoint than what is used inside Outlook. Here you are just working with the 'Shared Documents' library, but you can create many other document libraries for specific reasons. I guess if you can look through your organization current work culture and practices, then you would find many scenarios where SharePoint can come as useful.
I can think of few which would be useful, like, the modern interface and the modern web-parts. Then there is News, which lets you share your departments shareable items to other departments. You can have robust document management using specific content types and records management. etc. etc.