Library mail downloads Folder

Using Leopard on a G5 and moving to iMac with Mountain Lion.

What exactly does this folder do? It has photos, pdf's, xls files, etc. in it. Are these attached to emails in the inbox or to emails in the Mail/Mailboxes folder? Or are they here because they were opened and viewed when an email was read? I just need to know if they should be deleted before migrating to a new mac. I want all of the files so if they aren't attached to the original emails I'll have to keep them.

Please let me know if these are just copies or the originals.

Thanks