Recently we've seen major advancements in processor and graphics technology. While these will allow us to run the next generation of games, or speed up video-editing, CAD and other intensive applications, they also can consume gobs of power.
Dual-core CPUs, powerful graphics cards, and fast SATA drives all take their toll on a system's power supply unit. High-end systems can now require 700W PSUs. Even midrange gaming systems can demand hefty power supplies, and are more than capable of putting a significant load on them.