If you're doing PPC type things, then generally you have to pay your advertising bill reasonably quickly. But you may have to wait months for revenue from affiliate networks.

Also it's not unknown of for leads/sales to be disputed later on, or be reversed, or advertisers to go under, or affiliate networks to go under.

Also if you're paying for traffic in the hope of directing it to affiliate links, those affiliate links may be down/broken/messed up in other ways. So you have to be pretty sure everything is working exactly as it should be, especially if you're spending a lot on traffic.