A cold cover letter can help get your foot in the door of a company. It is possible that the hiring manager has not yet posted an open position that you happen to be qualified for, and they will consider you for the role. This letter can also help you expand your network and possibly lead to an informational interview.
Informational (or informal) interviews are basically casual interviews outside the work setting where you can get to know more about the hiring manager and the company they work for. Oftentimes, you can learn a lot of insider information about the company culture and how employees operate overall before you ever land the job, which can be extremely valuable in your job search.