Estimating market sizes
jeromec: I think market research companies use existing sales data to estimate current market size; figures can then be confirmed with light checking. The market size estimate can be helpful to provide a reference on which to base rough figures (which usually mean nothing) or hone an idea. Investors that may not understand an industry do understand numbers, so that's why such figures are popular. I'd keep in mind, though, that a market size can change up or down for various reasons.