like in most of P2P clients, the possibility to use Seeds/Sources is used to decrease bandwidth of the users and to increase availability of the files.
how it works ?
the seeds are identified with their own hashes (for example sha1, md5, crc, etc) and then checking if others users in the network are sharing the same file (comparing their hashes), so at the end (for example after a search) there will be more possibilities to download a file, even if a user that was originally sharing a file is not online, because will still be shared by others, also less bandwidth is used because is shared between the seeds/sources.
also the person that is downloading the file, is considered another seed/source, so can start share at the same time that is downloading the file.
will be good if something like this is implemented in Soulseek.