Our culture as a whole has evolved from being one mass market, to being a collection of lots of niches, subcultures, and genres. This is why research is important. No one will fully understand a subculture by just assuming. We’re all vastly different. We have to immerse ourselves in aspects of those subcultures to find the nuances and the identities of those groups.
Statistical data is helpful because it gives a designer/researcher a concrete view of measurable elements. It’s easy to transfer between people, and pretty easy to take at face value. This kind of data is in the realm of demographic information, surveys, and questionnaires.
Qualitative data is the hard part. It’s invisible, but palpable. It takes a variety of research methods to develop an understanding of the more abstract elements. This includes things like oral tradition, histories, responses, and attitude.
These types of information are helpful for design because, like in page layout (where it’s important to know the content you’re designing for), it just makes sense to truly understand the people you’re designing for.
Design is all about conveying information that is audience-appropriate. So, naturally, research plays a big role.