Rowing posture is very important. To be able to use your body weight properly to get leverage, protect your back you need to have good body posture. How you get there? By seating up, rotating your hips going to the catch and using the advance gravity center to load your weight at the catch.
Look at this video how the rower seats up all the time from the hips. It requires a lot of core and gluts power to keep that posture for a 90 minutes row non stop