It used to matter to Google what the domain name was for a topic. This was known as exact match domains (EMDs). So if you wanted to write a blog about bowling balls, you would have done best buying a domain name with those two words in it (i.e., bowlingballs.com). You also would have automatically ranked higher for the term "bowling balls." in Google's search results. Today, not so much. So while it would make sense to buy one related to the topic you picked, you no longer have to pick one to rank in a Google search. You could just as easily use some made-up domain name.