Having an ITIN number allows you to then ask Smashwords and Amazon to withhold less of your earnings for taxation, provided you live in a country that has a taxation treaty with the US (as many of them do.)
Please note the IRS is damned cryptic about how to do this sort of thing, so please pass this link along to anyone you know who self-publishes but is not a US citizen. It's very useful information!
How I Got My ITIN Number
