This is required if you have the Advanced "Skip Aggregation" option in the import step set to true (checked). When Skipping aggregation, this means that multiple files will be uploaded to the FTP endpoint. The Timestamp option is then required because the timestamp (so long as it includes seconds or milliseconds) in the filename will be different for each file that gets created by the import. Otherwise, the files that are being created on the FTP server will be overwritten one after another until the last file is written, causing a loss of data.
To remove this requirement, uncheck the "Skip Aggregation" option in the FTP import step.
A workaround is suggested in this community post, I hope this is helpful for you. If you have any questions feel free to reach out to us, thank you.
Regards,
Pridhvi