The Stretch Kit extends the distance between the front and rear Q-Towers thus increasing the crossbar spread and distributing the load on the roof. Most vehicles do not require the use of a Stretch Kit, those that do may have it listed as an option. If such is the case for your vehicle then the use will depend on what you are going to carry and the minimum required crossbar spread for that item, see the crossbar spread table.
If it is listed as a required accessory then you must use it. If you are unsure if you need a Stretch Kit refer to the Fit List here