What's wrong with Excel spreadsheets?
Well if you know exactly what's going to happen and their are no uncertainties in your data then Excel should be fine (if a bit clumsy if you have mutiple dimensions to handle but there are data base tool and business intelligence tools to help you with that).
Suppose, however, that you are not sure what will happen. Moreover, the next move may be determined by your own move. Competitive games are full of these issues and there are good game theory tools to assist people to make their choices (most notoriously in the recent auctons for 3G licences). Statistical theory can add quite a lot of value here by developing very rapidly a huge volume of possible futures and allowing the strategist to move towards an optimum. One recent well documented example shows how just 3 hours work running Monte Carlo simulations on a PC with off-the-the shelf software add-ons to Excel at a large airframe manufacturer bettered 3 months work of expensive by experts in halving the manufacturing labour cost. Think what this means in terms of increased flexibility and the ability to respond to the unexpected without unnecessary cost increases.
In a different vein, it is gradually becoming realised that Excel based decisions can lead to the wrong decisions on new product introductions, M&A, and a host of other areas where there is a time value to deferring the decision while more information is acquired. The whole area of real options needs the same tools but applied in a quite different way.