There was a time when football players played both on offense and defense. This became known as “iron man” football. It doesn’t exist anymore at the pro level, but one of the few ways modern-day players show their versatility is the role of the three-down linebacker.
Also known as an “every-down” linebacker, what makes these guys special is they are trusted enough, talented enough, athletic enough (you get the idea), to almost never come off the field whether it's first-and-10 or third-and-long.
The NFL is shifting to a more pass-friendly style of play, and with it defenses have to adjust.