A "what if" model is a more advanced use of spreadsheets. Spreadsheets were originally designed to automate the manual process of recording and tracking numbers. As the capabilities of spreadsheets grew, new types of applications became possible. The "what if" model is one such new application.
At an abstract level a "what if" model is more experimental and open-ended. You are generally simulating a transaction or process rather than calculating or aggregating data. More specifically, with a regular spreadsheet the calculations you code produce the final results. With a "what if" model the calculations you coded generally simulate some domain. The final results are:
For example, consider a spreadsheet that a CEO of a shipping company might create to determine the best location for a distribution warehouse. The input might be the state (for tax, labor, and other reasons), city (tax and relocation incentives), and location (distance to customers). Several examples might reveal that Branson, Missouri, is the best place to locate the new warehouse. The spreadsheet might also reveal that there is a financial incentive to locate in the southern part of the Midwest.
To summarize, with a "what if" model you are more interested in finding the right inputs to the model rather than the results computed by the model. The results computed by a "what if" model are important, but only to the extent that they show how the input values affect the domain.