A generation ago, the primary challenge offenses faced from the game's best pass-rushers was physical. At that time, both offensive and defensive players had more stationary roles and didn't move from area to area nearly as much as they do now.
In today's NFL, pass-rushers come from everywhere on the field, regardless of the down or situation. Just as 260-pound edge-rushers move inside to use their speed and agility to overmatch guards and centers, defensive tackles move out to the edge to physically dominate offensive tackles. And on passing downs (which is every down), a defensive lineman may kick back to linebacker depth and come at the quarterback based on the gaps created by the rest of the defense, much as a running back would follow the openings created by his offensive line.