What the hell does unnecessary roughness mean in the NFL?

I was watching Drew and Eli battle it out tonight. Sort of nodding off in front of the TV.

When you're nodding off the rise in crowd noise will roust you out of your stupor. When you're napping and there's a hockey game on, and the crowd roars, you know there's a fight.

If you're catching a few zee's while watching Man U and the Glasgow Rangers, and the crowd roars, you know someone almost scored. Almost being the keyword there. Nothing puts you to sleep like a European football game. No wonder their fans are fighting in the underground and on the trains. They just spent three hours watching their teams battle to a 0-0 draw.

There's absolutely nothing therapeutic about that.

The NFL on the other hand gives you unbridled violence,

So I'm snoozing in front of this Giants/ Saints game, and the crowd roars, and I gain consciousness just in time to see somebody getting a "unnecessary roughness" penalty?

How is such a thing even possible in the NFL?

In the NFL all roughness is absolutely necessary.

That's why I'm watching.