Joining a subquery is often used when you want to get aggregate data from a child/details table and display that along with records from the parent/header table. For example, you might want to get a count of child records, an average of some numeric column in child records, or the top or bottom row based on a date or numeric field. This example uses aliases, which arguable makes queries easier to read when you have multiple tables involved. Here's what a fairly typical subquery join looks like. In this case we are retrieving all rows from the parent table Purchase Orders and retrieving only the first row for each parent record of the child table PurchaseOrderLineItems.
SELECT po.Id, po.PODate, po.VendorName, po.Status, item.ItemNo, item.Description, item.Cost, item.Price FROM PurchaseOrders po LEFT JOIN ( SELECT l.PurchaseOrderId, l.ItemNo, l.Description, l.Cost, l.Price, Min(l.id) as Id FROM PurchaseOrderLineItems l GROUP BY l.PurchaseOrderId, l.ItemNo, l.Description, l.Cost, l.Price ) AS item ON item.PurchaseOrderId = po.Id