I built https://www.beatthatflight.com.au which works as a comparison flight/hotel search, as well as a deals finder website with an email subscription service. It's been incredible to see the changes on traffic, bookings and my revenue tanked 97% in April 2020 (good thing it's a side hustle) - Aussie centric website, so when the borders were closed, travel stopped.

The advantage of OTAs is you often can find cheaper hotels/flights than travel agents provide. However, some travel agents will then match or beat these prices.

The upside of a travel agent is when things go pearshaped. A lot of OTAs are based overseas and are (apparently) reluctant to deal with refunds/rebookings as readily, and you have to deal directly with them. Try speaking to the airline and they say 'oh you booked with an OTA, talk to them', and the OTA will say 'we're waiting to hear from the airline'. A travel agent will often do all this hunting/arguing for you, and can access Sabre/other GDSs to modify bookings themselves.

I've backpacked solo through most of the 'stans and South America. It's doable. But with a family, say? Or with a group of friends with fixed timelines (eg x weeks of annual leave) I'd be using a travel agent myself for some more adventurous travels.