It determines the content for your site and determines what it is talken about.
At first, your site should talk about your own field so you have to write on the topics you know it well and be aware of them, because if you didn't make this, the researcher wouldn't be attracted to your site.
If your field is photography and you are professional in photography and you have the ability to be the best in this field so you should design your site including photography.
If you searched on photography in Google and i find your site in the search result, when I enter your site and found it talking about another field like cooking so that is boring!
you should determine the content for your site, If you make an effort in your site and put all the topics that made the researcher on photography satisfied and when the visitor show up your site by the search engine Google and find that he searched about then your site will became the best in his point of view.
To sum up, what is your abilities? that is what you should know at the first place, that is will determine your domain name and content of your site and the keywords you will be targeted for on the search engine Google.