Cached data can be the only local copy available during travel, an outage, or after an account changes.
Before cleaning Mail attachments, confirm the message remains on the server or save the attachment to a permanent folder.
Before removing media cache, use the owning app to identify downloaded content and preserve items needed offline.
Consider bandwidth, account access, regional availability, and content no longer offered by its provider.
Quit the owning app, deselect important offline data, and clean a small batch.
Afterward, test the relevant messages or media while connected. Do not assume every service can redownload historical content indefinitely.