A higher daily limit
Guests get 10 conversions a day. A free account raises that ceiling, which matters most when you are working through a folder rather than converting a single file.
Conversion history
Signed-in conversions appear in your dashboard for 24 hours, so you can re-download a result without re-uploading the source. After 24 hours both the file and the record are deleted.
Larger batches
Free accounts can queue more files per batch. Results come back as a single zip rather than one download at a time.
What stays the same
Output quality, resolution and the complete absence of watermarks are identical at every tier, including anonymous use. An account changes throughput, not results.