Having adequate storage in the active user mailbox or OneDrive may not protect your critical data against accidental or intentional erasure, theft, or corruption.
In addition, Microsoft 365 offers Exchange Online archiving capabilities to extend the size of the Inbox. Coupled with features such as legal hold and retaining deleted items for some time, it may seem as if the data is protected.
However, it may not provide a complete, up-to-date copy of all the data and the granularity, searchability, and compliance capabilities that a separate data archive solution offers.
If your organization has specific regulatory or legal requirements for data retention and archiving, or if you need advanced search and discovery capabilities for historical data across user vaults, it may be beneficial to use a separate data archive solution for Microsoft 365. Learn more here