This is chapter 3, Become an Affiliate Yourself, of Successful Affiliate Marketing for Merchants.
The reasoning for an affiliate manager also being an affiliate is that they will have a 360 degree understanding of the industry.
I'm a little biased, because I was an affiliate before becoming an affiliate manager, but I think affiliates make the best affiliate managers.
However, and I would hope I don't even have to say this, but an affiliate manager should never be an affiliate for their own affiliate program.
Benefits of being an affiliate yourself include having a greater understanding of what your affiliates want and need from you. That will help you to serve your affiliates more efficiently.
Also, if you create an affiliate site in the same vertical as your company, then you can apply to your competitor's affiliate programs. This enables you to monitor their communications and types of creative.
Promoting the affiliate programs of your competition also enables you to gather competitive intelligence. By actually promoting them, you can get a feel for their conversion rates, EPC, etc.
You'll also be able to learn the dynamics of your competitor's affiliate programs by becoming their affiliate. From the first step of applying to their affiliate program, you can get an indication of their management style: do they automatically accept you as an affiliate or manually approve applications?
If it's the former, you might deduce that their affiliate program is on auto-pilot.
Finally, a merchant should never run affiliate offers on their own site prior to the completion of a transaction. Such cannibalization of your affiliate traffic will only serve to implode your affiliate program.
So anyway, go become an affiliate already!