Comparison sites charge travellers nothing and still operate large engineering and marketing organisations. The money arrives from the supplier side, and that changes what the results page looks like.
Two revenue models sit behind the results
Some sites are paid when a traveller clicks through to a supplier, regardless of whether a booking follows. Others are paid a commission only when a booking completes.
Click-based models reward volume of traffic sent onward, while commission models reward conversion, and the two produce noticeably different page designs.
Many large sites run both, which is why the same search can show a mixture of onward links and bookable options presented in similar visual style.
Ranking is not purely price order
A results page usually defaults to a blended ranking that considers price, but also duration, supplier reliability and the expected revenue from the listing.
Sorting strictly by price is normally available but rarely the default, because the cheapest option is often an itinerary most travellers will reject.
Understanding that the default is a judgement rather than a fact is the single most useful thing a traveller can know about these pages.
Inventory reaches the site by several routes
Fares come from global distribution systems, from direct airline connections and from consolidators, and each route carries different fare rules and refresh rates.
A fare shown from a slow-refreshing source can be unavailable by the time the traveller reaches the payment page, which is the origin of most vanishing-price complaints.
Suppliers that decline to connect at all simply do not appear, which is why some low-cost carriers are missing from certain comparison results entirely.
Agencies and suppliers are not equivalent
A booking made through an online agency creates a contract with that agency, and changes, refunds and disruption handling then pass through it.
A booking made directly with the airline or hotel keeps the relationship simple, which matters most when something goes wrong at short notice.
The price difference between the two is often small, and comparing them on total cost of a disrupted trip rather than the headline fare changes the answer frequently.
Using the tool rather than being used by it
Comparison sites are excellent at revealing which routes, dates and airports exist, which is genuinely hard information to assemble independently.
They are weaker as a final booking step, because that is precisely where the incentive gap between the site and the traveller is widest.
Searching broadly on a comparison site and then checking the winning option directly with the supplier captures most of the benefit with little of the cost.