I have heard some interesting stuff on various podcasts about agents roles. Firstly, in England they have way too much influence as most clubs rely on friendly agents to find them players. Which is batshit insane. Secondly, partly as a product of the first, they get a lot of big payments from helping clubs find exits for unwanted players. Clubs can set a minimum target for the value of a transfer, and tell the agent that if they make it happen, they can keep a % of whatever the sale is above that amount. So, if you have a midfielder you want out, you ask the agents to find him a new home, and tell him that they can have 20% of whatever over 10 million they manage to get for that deal. Find a club who will take him for 20 and suddenly that 20% of the extra 10 million is a 2 million payday for one deal for that agent. It's a really big incentive for agents to fuck about in the process.
And that's not even getting into third party ownership, image rights and organize crime.