There's always been a competition between armored units and infantry — well before the Javelin missile. As far back as the Middle Ages, developments in technology constantly shifted who had the upper hand. For example, gleaming knights of old wore heavy armor that protected them from most weaponry — at least until the Battle of Agincourt introduced the piercing, infantry-wielded English longbow. Throughout history, technologies developed back and forth, until, finally, the gun firmly established that an ordinary grunt could beat armor with a good shot.
However, World War I drastically changed that dynamic. The tank emerged as the modern equivalent of armored knights, seemingly untouchable by infantry. The armored edge continued to grow through World War II. Even with the development of the bazooka, the best way to kill tanks was either with other tanks, or to call in artillery or air strikes. Times were tough for infantry.
The development of the FGM-77 Dragon and the BGM-71 Tube-launched Optically-tracked Wire-guided (TOW) missile helped American grunts, but these still had problems. First, the wire guidance meant that anti-tank teams had to stay in one location to guide the missile. Any sudden moves would put the missile off course. As you might imagine, remaining stationary in the face of a tank isn't a great idea.
Second, the missiles had a huge back-blast, which would immediately alert enemy armor to the idea that they're being attacked. This, coupled with the wire guidance, meant enemy tanks knew when and where to look for anti-armor specialists. TOW teams were lucky: The missile's range of 2.3 miles allowed the crews some standoff distance. Folks with the Dragon, sporting a range of just under a mile, often found themselves within heavy machine-gun range upon firing.
Thankfully, these issues have been addressed with the introduction of the FGM-148 Javelin. With a maximum range of about 1.5 miles, it gives the crews the ability to stand off. More importantly, it's a fire-and-forget missile with a much-reduced backblast. So, even if the launch position is detected, the team can move to a new location, leaving enemy fire to rain upon an empty foxhole. The missile can attack the top of an armored vehicle (useful against tanks like the Russian Armata) or carry out a frontal attack.
That is why the Javelin is so deadly: It gives the light infantry a fighting chance against tanks. When you consider that "light" units, like the 82nd Airborne, are usually followed by heavier units with lots of tanks, the Javelin's importance becomes very apparent.