It all depends on the power budget of the device as well as the stepped down voltages the device requires. Apple laptops, for example, require a number of different voltages: 1.0 or less for the CPU core, 1.8, 2.5, 3.3, 5, sometimes 12, etc. It's much easier (and more efficient) to step down to a lower voltage than step up to a higher one.
Also, with Apple laptops, the input voltage is also used to determine which type of power adaptor is connected, either the standard 65 or 85 W adaptors (to determine whether you have enough power to charge the battery while running the machine) or the airline adaptor (which can't charge an Apple laptop). A number of voltage comparators on the input perform these determinations.
I can't say much about any of this for other computers, but 19 V sounds like a high enough voltage to produce all these lower voltages easily.
Steve
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