People told me I should reach out to potential postdoc supervisors about 6-12 months before I finish my PhD and stay in touch with them.

But what should I say when I email them? Should I just ask them if they plan to take on a new postdoc this year? But they don't have an ad for the recruitment, which clearly shows that they don't have any money at the moment for a new postdoc (if they had the money, then they would post an ad for sure!). So asking if they are recruiting a postdoc sounds like a stupid question. And if they do have an ad, then I should just submit an application - what's the point in reaching out to them before I submit an application?

And what other things can I say in my emails to them? Maybe I should introduce myself? But if they don't have money for a new postdoc, it should be meaningless to introduce myself I think?

I really don't understand why people keep telling to start early and reach out to the academics I'm interested in. I don't think I have anything to say to them.

Things are not great in academia when it comes to funding. If a PI has fund, he/she is "more likely" to post a job ad. Some PIs have some "hidden" funds or they have money from department which is not assigned to a particular project. You might try though the chance is not particularly high.
One expected reply is that PI asks you for a proposal either his/her proposal and you want you to work on it or your own proposal. Many academics think now that the Postdoc and even PhD students should bring their money with them.
Another important option is to seek fund from funding bodies that are willing to fund early stage researchers. The PI can guide you which funding bodies you can contact or you might start googling it before contacting PIs.

Hi sciencePhD, I haven't reached out to any potential supervisors myself. Though I thought you could email them saying, I read your papers on X and it compliments my work on Y so do you want to work together on Z. If they are interested in your work they might talk with you more and pay off. I understand your hesitancy to email people because of funding but funding can always be acquired after your first contact. Ideally if you contact them you will become their first choice for a later postdoc without having to work about the application process.