Simple. It doesn't. I suppose in the instance where someone is using 24 gauge wire it might help, but using one 18 gauge wire would do exactly the same thing. This is a great example of lay intuition. The industry is perfectly willing to support it because they get to sell you another stupidly expensive cable.
Now, bi amping is another issue. This is where you delete the passive, analog crossover entirely and use an electronic analog or digital crossover and separate amps to drive each driver. This can have huge advantages if done correctly. It requires careful amp selection and crossover programming.