Every once in a while I worry about advocacy against a strawman argument.
Not a lot, mind you, because a major purpose of this site is to catalogue lots of trivial bits of polite society. But if you want to persuade people, you might as well persuade someone that exists.
But I'd say there are some important examples where you should persuade people of something even when nobody really disagrees with you. And that's when you're talking about actual extant laws.
We should stop minting pennies, in America. That's been true for a while and only gets more true every year. No serious person thinks otherwise (the latest attempt to abolish the penny, to my knowledge, was from the late John McCain). But because people aren't fired up, it doesn't happen. Which means, to accomplish simple high-functioning governmental purposes we need to get people revved up about changing the law as it exists – even when there's no one to argue against.
This also applies to lots of other laws crafted in an age we've long since left – The Espionage Act, for instance, involves an element of society it's best to do well, so we should probably have a law that doesn't totally suck. The PATRIOT Act keeps getting renewed, for reasons passing my personal comprehension, as well as the AUMF drafted in response to 9/11. Well, we won, bin Laden is dead, and I think every normal person would be on board with more honest legislation about this. Weekend voting has been discussed endlessly, or at least making election day a national holiday (for national elections, of course – local elections could have local holidays – do those exist?).
There are also suggestions about simplifying the tax filing process, as well as giving people receipts for where the money went – these would not be expensive to do (truly rounding errors) and be pretty profound public services. There are some well known boondoggles as well (the F35 project is honestly rage-inducing) – I've never met someone who has defended them, but presumably they get support in Congress.
There is a part of me that feels we should pass a bill stopping the minting of the penny and personally awarding every member of congress $10MM USD. We'd still come out ahead, and I'm sure they'd love it.