Human-Computer Interaction is the study of how to conceive, build, and analyze systems people use. Building something good takes work and requires a good understanding of the user's needs and workflows. Whether the industry is healthcare, finance, commerce, transportation, information, communication, work or play, it's important to work from data to verify that you are addressing the right problems.
Diagnose the situation, determine the time available for research, decide on the right research questions, choose qualitative and quantitative research methods, pursue. Communicate findings by: usability aspect reports, videos, presentations, and/or mock-ups.
Still have doubts? Wondering what the big deal is about user-centered design? Read "How to Get Your Coworkers to Care About User Research."