I'm not sure how it is in the US, but in Canada a huge amount of low-income benefits are directly tied to filing your taxes. Most Canadians experienced this in our college years when we got GST (our VAT) refunds due to being low-income adults.

Canada recently announced that they're going to go for automated tax filing and it turns out the biggest cost may not be implementing it, but that they'd end up having to pay out a lot more in benefits to low income people that don't file.