With its lower hook the seat harness will transfer more of your weight from feet to mast base. It helps keeping the board flat on water, moreover if you wear a weight jacket
Went from 15 years of "sitting" and adopted the waist in a minute. Just found it more convenient & comfortable overall although you get tired faster in overpowered conditions. However if your difficulties come from poor rigging/tuning a waist harness may not fix them, but it can make you more confident which is 50% of the job.