Well, even if nobody else seeds you're still no worse off than you would be with just an HTTP server. At present the main incentive is generosity, which works pretty well for a lot of things; for example a lot of people started pinning the Turkish Wikipedia mirror when it was announced purely as a stand against censorship. In addition, you can also pay a pinning service. In the longer run the IPFS developers are also working on Filecoin, which is a cryptocurrency that adds a payments layer on top of IPFS so you can pay to have someone else serve your content.
Ethical/moral interest in the content, for one (wolfgang42 gave an example of that). That's a pretty small market though.
The other content could be academic and an alternative to information akin to the old Usenet (pre binaries) and things like Gopher and BBS.
The major demand for a system like this, I'd wager, is pirated content. That is, if its better than the current models. The two big models currently being used for piracy are:
1) Usenet/NZB
* Centralized. Only a handful large networks with long retention. Rest are resellers.
* Usenet servers adhere to DMCA; indexers generally don't
* Pretty much requires payment for payserver & indexer
* Little bit rougher on resources due to PAR2 and quick downloading/unpacking of large files has I/O impact. Nothing huge on recent desktop systems.
* Requires a bit more software for everything to work though nowadays there are all in one packages doing the dirty work for you.
2) BitTorrent
* Decentralized.
* Not anonymous. Requires at least VPN, who likely log, even though they claim they don't.
* Free as in beer
* Requires upload
* Based on open source and open standards
* Self-hosted. Some indexers adhere to DMCA; generally
they don't
* Private indexers ("trackers") have a reputation system requiring more than 1:1 download/upload.
* Doesn't work at all with DSLite (native IPv6 + IPv4 behind NAT).
* BitTorrent, like Usenet, also has abstraction software (full stack) to make life easier.
BitTorrent is far better known among the general public but I regard BitTorrent as a poor man's Usenet. If it ain't on Highwinds, it ain't anywhere (of course not completely true given DMCA but still, they got 3,3k days retention).
With Usenet you pay for a sub to a payserver & indexer, with BitTorrent you pay by uploading and VPN.
Now, my question is, where does IPFS belong in this list? Can it compete with these 2 technologies?
