Many reasons. A person is capped by 2 offensive spy attacks per day, and might not have enough spies at the moment. You want 60 spies and CIA for maximum chances. If you are a person who wants to spy out more nukes than 2 per day, and want anonymity, it is a great option.
A side benefit is use during wars. When I was running spy ops in Mensa HQ, I would have to donate the exact cost of the spy op to every member. Through this system, an alliance can just post bounties on the people with nukes, and the person who kills the nuke gets immediate recompensation rather than having to contact an alliance operative and prove that he or she did indeed do the op. This also helps recompensation to happen across allied alliances.
Given that we have bounties for raids and attrition, I think it is only logical.