Having the power to hire and fire in a company is akin to having power over life and death.
Your thumbs up means the pimply Gen Y-er gets the job as head of the tech department, a job he's dreamed of having. A thumbs down, and he's stuck with doing QA testing on micro-devices. HR directors can throw their weight around, weighing in on company policy, compensation levels, health, safety -- in fact, on every aspect of the corporate body politic.
If that's not clout, then what is?