I spoke to Google about this last year due to the ominous warning appearing. They checked my ads.txt and said it was fine after which they said it was a bug in the systems which should disappear in few days.
Ads.txt is basically there to confirm you are the site owner and approve adsense to run on the site - the idea being that the site owner should be the only one who can upload the ads.txt
If you run many sites you could also run adsense through those sites and in turn should verify they are yours.
The problems start when scrapers take your content and your ads appear on their site. You as it's your ad you'd get the revenue from it. So this is one way to hamper such things - and increase Googles revenue from illicit clicks - which in turn is meant to protect you.
Finally there's an option on many sites to include your Adsense code to get a share of the revenue on content you've produced. Blogging platforms or some wiki style sites allow the content creator of the page to have an adblock with their adsense code in it as a reward. Again, the ads.txt would be needed here but can't always be implemented due to the nature of these sites and their set ups.
As a note, the ads.txt warning took about a month to disappear. It's back again this year and I've completely ignored it. If your earnings disappear and you have this warning then you should indeed send them a message and check out your ads.txt.