I got curious and looked it up. I found this[0]:

> As the storage miner receives each piece of client data, they place it into a sector. Sectors are the fundamental units of storage in Filecoin, and can contain pieces from multiple deals and clients.

> Next, a process called sealing takes place. During sealing, the sector data is encoded through a sequence of graph and hashing processes to create a unique replica. The encoding process is designed to be slow and computationally heavy, making it difficult to spoof.

[0] https://blog.coinlist.co/deep-dive-into-filecoin/

Oh, so it's secretly proof of work.

I think it's worse than that; it looks like Filecoin's PoRep proves that multiple copies of the data are stored on the same server. I don't understand why this is useful.

There's no incentive to store multiple copies of the same data vs a single copy of multiple datas. It's a possible attack vector, for example you could store all of the copies of some data and then lose it, but there's no economic incentive to do so from what I understand.