By selecting a matrix and choosing "Insert Table" from the menu, you create a table which allows you to pull and insert data in a structured way. Let's say you have named the table "SalesEvents" and given that the first (header) row reads "Salesperson" "Date" "Sales Amount", you can calculate sales amount like this:
=SUM(SalesEvents[Sales Amount]). Entering data at the bottom of the table automatically adds new rows. This is a very good way of enhancing structure by encapsulating deltails inside your tables, leaving outside cells good posibilities to extract the high-level properties like column sums.