The markers were thought to be some higher organisms blue print of life.
There are two markers.
One is the original black one found on earth, the other is the red one encountered in the game.
Black one the scientiest tried to copy. In the process they got the red one. They also attempted to get the dna code from the original, but it turned out to be a recombinant virus that made people necromorphs.
The red marker was mostly useless because it made people crazy BUT it could stop the necromorphs from going apeshit.
Hence when the necromorph invasion happened originally, the government buried it at the time using the red marker. Hence its constant whining about making it whole again.
In short necromorphs motivations are obvious. They want to be free and eat people. The markers motivations are the same as nicoles, she wants to be whole again to stop the necro's from going ballistic.
Basically they let you fill in the gaps early, and explained it a slightly different way later.