Why is it so important to do this research? First, it shows you were willing to go above and beyond to get your resume into the right hands. The hiring manager will probably think you’re resourceful, have a strong network of contacts, and are truly interested in the job. It’s not always easy to find out where the posting came from, so just this small personal touch will impress the hiring manager and set you apart from other candidates. Plus, by finding a direct email address rather than a generic address like firstname.lastname@example.org, your resume won’t get lost in the endless stream of submissions that flow in every time a posting goes wide.
Secondly, knowing the hiring manager’s name allows you to personalize your cover email. The best cover emails are conversational, so starting one with “Dear Hiring Manager,” puts you at a disadvantage. Sometimes this is unavoidable -- not all postings provide enough clues for tracking down the company name -- but if at all possible, try to find a human to send your resume to, and start your cover email with “Hi [person’s name].” Then, continue on with a very simple explanation of why you’re right for the job, and you’ll have at the very least earned yourself a resume download. Plain and simple -- if you want to be taken seriously, personalizing your cover letter is a very easy way to get off on the right foot.
Ultimately, identifying the hiring manager will help prove you want the job and improve your likability factor. Not only will you have demonstrated enthusiasm by doing extra research, but you will sound far less robotic in your cover letter. Although this step may seem like a small detail in the overall application process, you're giving yourself an edge over other candidates, and that's always a good thing.