Uploading a Mind Map to a server has several advantages:
It allows you to share Mind Maps with other SimpleMind Touch users (via email links). After uploading you get the option of sending an email link. Opening this link on an other touch device starts SimpleMind and imports the Mind Map on that device.
The server acts as a backup for your Mind Maps. Even if you loose a Mind Map on your device, you may restore it from the server.
Details can be found here.