I have no clue why anyone has used one in atleast the pass decade, but I can remember when it was a thing because the internet didn't exist. Even printed guide books would be at least a year out of date the day they went to press... booking travel to an area you didn't know was...difficult.

In EU you will end up under Package travel directive. Which means extra protections, and single point of communication. Like when COVID hit and travel recommendations were issued. My whole holiday of hotel+flight got cancelled, and I got automatic refund for whole thing. Just had to give them by account number.

It makes life simpler in cases when flights or hotel gets cancelled and agency is responsible for handling things.

There are times when booking through Ryanair over their app gives €X price, but through some 3rd party like Kiwi I might find e.g. €(X*0.5).

However, even though I almost always fly with just a small bag, I still might hesitate taking the 3rd party service because they insist on doing stuff like checking in "for me", which I don't need (since that is a trivial process), and I have experienced them not sending the boarding pass in the time I arrived at the airport.

It's nice that they are cheaper until you realize that you've booked a connecting flight through Kiwi and then realize that Ryanair doesn't offer that service, so you'll be picking up your checked luggage at the layover and the queuing again to check the luggage for the next destination. You miss the connecting flight, good luck with Kiwi. They're notorious for their poor handling of this exact scenario, since the money will have to come out of their pockets. Ryanair can only be held responsible for delays of the first flight.

Sometimes they do search on travel agency website and accept the option, not realizing that at this point they could have booked directly from an airline's website.

We use TravelPerk to book and pay for the entire process of biz trip (travel,hotels etc).