You shouldn't trust public services to protect your data from eavesdropping, and this is no exception. Encryption guarantees that nobody except you and the desired recipients can see the transmitted data. It makes also possible to check the authenticity and detect possible tampering.
This is not a backup service. Uploaded files can be deleted at any time. There is a limited time on which the files can be available for download, but it is not guaranteed that the files will be kept for that long.