Claim companies work on no win, no fee. If they recover your compensation they keep a commission, typically 25 to 35 percent, and some add VAT or a separate court fee on top of that.
On a €600 claim that is €150 to €210 of your money. For a family of four it is €600 to €840 out of €2,400.
Nothing about that is hidden or dishonest. The question is simply whether the work they do is worth that share, and the honest answer is that it depends entirely on your airline's behaviour.
When it is worth paying
The airline has refused and is stonewalling you. If you have already sent the second letter and got nowhere, a claim company has the volume and the legal muscle to push, and they will go to court where you might not.
The airline is genuinely disputing the cause. Cases turning on whether a technical fault was extraordinary can need evidence you cannot get. They can.
Your claim is old or the paperwork is gone. They can often reconstruct flight records you no longer have.
You simply will not do it otherwise. Sixty five percent of something beats one hundred percent of a claim you never file. This is the real reason most people should use one, and there is no shame in it.
When it is a waste of money
You have not asked the airline yet. A large share of valid claims are paid on the first email. Handing over a third of your money before finding that out is the most common mistake people make.
The cause is clear cut. The airline told you it was a technical fault, you arrived four hours late, the route is straightforward. That is one email, not a case.
The claim is small. On a €250 claim, a third is €82. The same email either way.
The honest sequence
Send one email yourself first. Give them 14 days. If they pay, you have kept the full amount for fifteen minutes of work. If they refuse, send the second letter asking them to substantiate it. If they still refuse, then hand it to a claim company or escalate to the regulator, which is free.
You lose nothing by trying yourself first. The time limits run to years, not weeks, so a fortnight spent asking costs you nothing at all.
Disclosure: that is an affiliate link and this site earns a commission if you use it. It has not changed a word of the advice above, which is still that you should email the airline yourself first and keep all of the money if they pay.