I'm not referring to travel agencies in general -- here I narrow it down to travel agencies that sell airplane tickets.
For hotels, it's pretty much understandable to me, as I always prefer e.g. booking.com over filling out some lousy booking form of a certain hotel. The middle man here has a nice tool to visualize the hotel position, to compare the prices. They has better interface; reputable payment system, etc etc.
But when it comes to air ticket booking, I wonder why booking through agencies is still a (common?) choice (e.g. for each search in Skyscanner, dozens of agencies come up with their listed price).
Pretty often, the "cheap" tickets as advertised by the agencies turn out to be anything but. Many people complain about the hassles they had to go through with the agency (suddenly cancel the booking, no actual booking found in the airline's system, no support lines, or changing the trip seems impossible, etc.). Perhaps this is biased because people who went through without any problem would not bother to say "oh hey, I did it with them, perfect, no problem".
But even when considering the price, it's not the selling point of the agencies as well. Personal experience today: I was searching for the Paris - Osaka ticket. Some agencies' advertised price may sound 60-80€ cheaper, but going step by step to the payment page, an agency normally adds lots of fees, and adding the unavoidable "administration/document fee" makes the total maybe equal (or even more) than the price if one was to buy at the airline selling site.
Unlike hotels, seems that almost all airline companies have a decent selling web, and I don't understand how booking through agency is "easier". Either way, you'll have to go through steps where you fill the name, DoB, passport number, phone number, address, and pay - how hard can it be?
Skyscanner is living well because the agencies are living well. So what is/are the agencies' selling point(s)?
For me I only use SS as a search engine for reference, then go directly to the airline website.
Sometimes (not many times) buying via agencies can actually be quite cheaper, perhaps they have quota to sell in lots/batches, but it happened very rarely to me.