This is a great explanation, but you're missing the reasoning behind selling off the debt.
Acme Corporation wants the money back. The problem is that if they want to get the debt back, they're going to have to spend time and money to get Bob to pay. Often, they won't even get the money back, so it doesn't make sense to spend a lot of money in getting Bob to pay when they might not even get the money back. What they decide to do is tell debt collectors, people devoted to collecting debt, that for some amount of money (for example $5 on $100 debt), the debt collectors can get the rights to collect the debt, and if they are able to get the money, then they are entitled to some of the money, while some goes back to Acme Corporation.
This leaves Acme Corporation able to do what they usually do without having to deal with collecting debt while at the same time still getting some of the money back.
Bob owes Acme Corporation $100
Acme Corporation pays Company Y $5 to collect the $100 from Bob. Acme Corporation can now focus on other things.
Company Y collects the debt. They give some of the money to Acme Corporation and the rest they keep.
Acme Corporation now gets some of the money back. They didn't get all of the money, but they got some of it. If Acme had tried to collect the debt themselves, it's possible that they spend more than they get back.
For the lunchbox analogy it works pretty well. The lunchbox has $100 in it. You would need to spend a lot of money to get it open, and you would have to focus on opening it rather than doing other things, so you tell someone else that they'll give them the lunchbox for $5 and if they are able to open it, then that someone can keep some of the money.