Knowing your market is an important aspect to writing a query letter. In order to sell the article, you must know that it will fit the market to which it will be submitted. To be sure, you should study several recent back issues of the market you wish to query. Read through the magazine, noting the type and length of articles, editorial content, style, departments, number of photographs with each article, the names of the editorial staff, and the types of advertisements. Advertisements are a wealth of information when studying a market.
Companies spend millions of dollars to target their selected markets.
When you look at an advertisement closely, it will tell you the readership of the publication in which it appears. That is the readership of that particular publication. When you know your subject and market, your query will be well placed, and therefore will have a better chance of being accepted. All that's left is to write a query that grabs the editor, tells that editor exactly what to expect, and why you should write the article. Grab that editor.