What role does audience research play in shaping a long-term blogging strategy?

Having the right idea about audience, the way they think, the way they operate, what they need etc is the centre of post. It is the most crucial part. Basically the grounds of your post.

audience research helps you understand what people actually want to read . it helps choosing topics tone and type of content to create. over time this can bring more relevant traffic and build loyal audience

It's core principle of blogging in my opinion. Once you know your audience, you will create better content according your audience interest. They will get a content that provides some value. Your engagment will grow along with your authority in the domain. In long term you can also create a brand out of this. so research your audience before anything else.