Leverage is a key factor of rowing.
If you lever well you get more load so you get strong.
You have to connect the blade to the water and your body to the boat trough the fotstretcher to be able to generate the biggest amount of force you can on the oar handle.
To connect your feet to your hands the work has to travel trough your body. In order to do that efficiently you need to be really strong in your core and in your hips.