Say the manager is looking at a handful of resumés and portfolios, and each of them present candidates who are accomplished writers who could do the job. But your resumé presents you as a silo-buster, able to do more than the job at hand but many of the adjacent tasks that might come up. That represents real value to the employer since he or she will get the skills of several employees for the price of one.
This is almost exactly identical to what I mentioned earlier. When you are well rounded and have skills in many areas, you are more beneficial to a company and therefore more people will want to hire you.