Major topics are like genres. One of the ways that they are useful is that they help subdivide articles into categories. They can be particularly useful when you are trying to research an unusual aspect of a topic (such as if you wanted to analyze the pieces of a jazz composer, or if you wanted ethnographic information on a classical composer).
Let us start by looking at this item again:
Its major topic is "29: Western art music--History. 1910 to present (musical life)." If we wanted to find more items with the same major topic, we would add it to the search by typing it in the second search box and changing the field to "MT Major Topics."
From the Advanced Search screen, you can also select a range of major topics: