Filecoin is a mechanism for someone to pay to ensure that data stays available. So if a group of people wanted to ensure that myoldwebsite.com stays available through IPFS and the IA, they could spend Filecoin. The difference between it and paying some central provider with fiat is that anyone can provide the data availability. So if you don't trust IA to be the long-term provider of content or want a more decentralized provider, Filecoin lets you do that. See also Arweave.