The story of how technologies become platforms is important, and Joel on Software on commoditisation explains it well with plenty of examples. In brief, 1 "A complement is a product that you usually buy together with another product"; 2 "demand for a product increases when the price of its complements decreases". So a company should make its product's complements a commodity, that is, interchangable, because then the competition in the complement's market is maximised and the price is minimised.
In practice: "When IBM designed the PC architecture, they used off-the-shelf parts instead of custom parts, and they carefully documented the interfaces between the parts in the (revolutionary) IBM-PC Technical Reference Manual. Why? So that other manufacturers could join the party. As long as you match the interface, you can be used in PCs. IBM's goal was to commoditize the add-in market, which is a complement of the PC market, and they did this quite successfully. Within a short time scrillions of companies sprung up offering memory cards, hard drives, graphics cards, printers, etc. Cheap add-ins meant more demand for PCs".