Joint ventures.
Keeping the traffic from completely different networks of sites or promotional methods completely separate.
Accounting trickery if you are into tax avoidance / evasion.
A hedge against getting banned if you are using dubious promotional techniques (obviously you need different names etc for this option).
Another option might be if you are promoting some stuff that is "high risk" and gets a lot of refunds, and other stuff that is lower risk and doesn't get a lot of refunds. If you get a lot of refunds then CB will withhold more of your funds as the "allowance" and as far as I am aware this is determined on a per-account basis. You could segregate your high risk stuff in its own account; the stuff in your low risk account would have less of an allowance withheld, giving you faster access to more of your funds.