So, if you divide up "things you sell tickets for" into "things that will easily sell out" and "things that aren't going to sell out", and accept that you need to treat them differently... where do agencies fit in?
It should be fairly clear that a ticket agency can be very helpful with a show that isn't going to sell out. They can be given an allocation, and that doesn't matter much because there is less competition for the other tickets. They can provide additional marketing, and they have a list of customers which spreads across venues and promoters, so they can reach customers you otherwise wouldn't be able to reach. They can offer tickets to shows across a geographical area and become a portal, so that the casual visitor can see what's on and be directed towards tickets without having to check each venue. They add value, from the promoter's point of view, and it would be reasonable to give them a commission on the tickets that they sell that wouldn't otherwise get sold.
It should be equally clear that a ticket agency doesn't have much to add with a sell out show. If it's popular, has an established fan base, has rave reviews, people will know about it anyway, without the benefit of marketing. Adding an extra layer to the sales process makes things less efficient and more expensive. And should anything go wrong, necessitating a refund or a cancellation or an exchange, the promoter can't do anything about it because the agency took the money, and the agency won't do anything about it because they don't care. If an agency gets an allocation to a show that would sell out anyway, then once all the tickets remaining at the box office have been sold, the agency will have a monopoly on the remaining tickets, and commission notwithstanding can charge whatever booking fee they like on top of the "face value" to the people scrabbling for the last few tickets. They make life harder, from the promoter's point of view, but can make a lot more money a lot more easily from the agency's point of view.
There are a few agencies that specialise in "shows that won't sell out" - the TKTS booth is the first that springs to mind. And there are many many more that make a living out of touting those last few tickets to sold out shows, adding nothing of value and promoting the impression that "booking fees are a rip off" - because in this case, they are.