Perhaps It is because I did STEM in my undergrad, but this is so incredibly important to the LSAT and the actual law. Say an expert witness for a case discusses that X was caused by Y as determined by an experiment. If they did not account for Z, and Z occurs in this case and could be correlated to the outcome of Y, how could we conclusively blame X for causing Y? Z Could very well have caused Y!

So if we were to weaken an argument that X caused Y, and one of the options was discussing Z, which was also present and could lead to Y, that would significantly weaken the argument that X caused Y