Power
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?