Thigh straps are essential kayak accessories if you paddle your sit-on-top kayak on the sea, rivers and rough water. Thigh straps connect you to the deck so that waves splashing over the deck will not lift you from your seat. If you do capsize, your knees will immediately slip out to free you.
Using thigh straps gives you better control, allowing you to edge the kayak using your lower body. This is especially useful on the sea and going down low-class rapids.