You also only need to fill in a tax return if you have income (or capital gains) above a threshold. SO having some interest paid on a savings account etc or a small side business or selling an asset at a small profit above what you paid for it does not mean you have to make a tax return.
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.
