The fullback, aka the left or right back, plays a very crucial role in modern day football. It is not to say that it never played a crucial role, it suggests that in today’s day and age, the fullback is given much more room to maneuver, but much more responsibility as well.
In the modern day 4-4-2 formation, the fullback has very little support from a defensive standpoint. Because the defense is arranged in a flat line and the two central defenders have the responsibility of marking the two forwards, the fullbacks are left to fend for themselves in many regards.