I have not personally used these websites, but it is possible that these are legitimate websites.
In the EU, there are strong air passenger protections. From that website:
In addition, if you are denied boarding, your flight is cancelled or
arrives more than 3 hours late on arrival at the final destination
stated on your ticket, you may be entitled to compensation of €250 -
600, depending on the distance of the flight.
Naturally, getting payment for such a claim is made exceedingly difficult by the airline: they will repeatedly refuse to pay legitimate claims. A friend was flying back to the US from Spain and had a flight that was delayed for 24 hours for non-weather reasons and registered a claim. He made repeated requests for payment and followed up with both Spanish and EU regulatory boards regarding the matter, and the airline still refused to pay. Eventually, his hounding caused the airline to offer him compensation, but he held out for the full 600 euro he was entitled to, and received it, but it took more than six months of back-and-forth with the airline.
I was booked on the same flight with my friend, but did not follow up with the airline, as the amount of paperwork and repeated requests required was more effort than I cared to put in. This is where these websites would presumably step in, and handle the negotiation with the airline on your behalf, in exchange for part of the proceeds.