Be subtle! be subtle! and use your spies for every kind of business.
Spies are delicate tool. Crass use will expose them and ruin you, so be very careful.
Spies can be used very creatively. You can use them in your own camp to seek out dissenters. You can use them with allies, to check on their loyalty. You can place them amongst the common folk to understand and nudge local populations.
Well-used, spies can plant and provide information that catalyze and tip the balance. With small actions they can make big differences.
Spying in business has the additional problem of possibly being illegal. If you step into this gray zone, you will need to be additionally careful.