It's nothing but a great big rip-off - often exacerbated by theatres that know you can't buy tickets for the show and venue from anywhere else.
I think it started in the 'olden' days before the internet, when you had to have physical printed tickets and would thus have to either go and collect them from the box office (travel costs, parking costs, the time to get there during their opening hours etc.) or otherwise, for your convenience, they would 'kindly' pop them in an envelope and send them to you.
For the latter option, they would always charge you for the postage, the (suspiciously expensive) envelope and their admin/handling fee, which usually included a very generous 'bonus' for them. Once tickets started to become downloads that they would press one button to send to you and then you would press one button to download them, they were very reluctant to give up this extra bonus, so they just kept it: extra money for absolutely no reason whatsoever. Many of them will even charge you a fee per ticket, even though, in the old days, posting you six tickets would cost no more than sending just one.
It's utterly disgusting, but people have now become inured to it and expect it - a bit like in restaurants that already charge you a price to reflect the experience and convenience of all you get as standard from a restaurant (obviously it's way, way more than you'd pay to buy the food from Tesco, cook it and then do the washing up yourself), but then add on an extra 'service charge', often also angling for a 'tip' on top of that!
Personally, I always just want to know the full price of getting the goods/services from them. I don't care about how much it costs them in their overheads or with their suppliers - they run their business, not me; I just want them to work out all of their costings, margins and desired profits, so as to decide what they need to charge me as the end consumer, and give me the price so that I can choose whether or not to buy it.
Ironically, most of them would probably engender much less ill will if they added the 'booking fee' price on to the ticket face value, and most would pay it without a quibble; but they would probably then, sooner or later, still add another booking fee (on to the price that was already increased to include the former booking fee) as they just will not let their golden goose die.