This saying makes a lot of sense because no plan, no matter how nice it sounds, has any effect until it is implemented.
If a manager is only good at making plans, he or she will not really be very effective. These managers can devise great strategies that will really help their firms prosper. However, if the strategies cannot be implemented well, or are not implemented well, they will simply be pieces of paper that look good, but are of no use to the firm.
Strategies in business are worthwhile only when they can be translated into actions in the real world. Therefore, a strategy is only effective if it can be implemented well.