My understanding is limited but I understand that you can make a zero payment but still do the proof of work on 2 prior payments on the network. So it is essentially free work done for the benefit of others, not yourself (although you may see payments confirming quickly on the IOTA network as a general benefit).
The idea is that payments get confirmed more quickly. But it is unclear whether the full nodes can handle this sufficiently already (many light nodes, less full nodes out there). So this user initiative is also a good test to see how good IOTA can handle 'spam'.