Office 365 does not have a comprehensive backup solution. Once the retention period expires, data will be deleted and cannot be recovered.
Protecting yourself against ransomware attacks
Microsoft expressly states that recovery of Exchange data at a certain point in time is not possible. Keeping a separate copy of the data makes it possible to recover quickly from an attack.
Planning an exit scenario
Planning exit scenarios from the beginning should be the part of every business decision, especially when it comes to your own data.
Avoid dependencies & blackmail
A company that is completely dependent on a single cloud provider is at a great risk. People are even trying to blackmail companies.
When users are deleted from Active Directory, all of their data and associated SharePoint pages are deleted. What happens when they are needed for a lawsuit?
Nowadays, companies are demanding that roles be kept separate. Doing backups on the production platform creates a single point of failure. It is possible to delete data in Office 365 either accidentally or wilfully.