Specifically designed harness to be mounted at six strap loops extra strong, which need to be purchased seperately first as long as the boat is not prepared for (like the MRS Alligator 2S).
The harness has a three point setup. The third location directly to the knee (delta strap), making the shape of the straps a triangle, results in a stroke much more responsive to the whole body of the boat as well as the distributing over the fabric (avoiding a leverage stress).
Thigh straps are primarily meant for effective control in the boat (by bracing and edging), up to the point of rolling it. However, they also are a good shoulder padding for carrying a partly packed boat. It transforms a packraft into a backpack itself. Being a kayaker by origin you will hardly quest the use of thigh straps.
Being a beginner however, one should not overestimate its function. Packrafts in general are primarily designed for use without them. We recommend to start that way. Up to medium grades of white water there is no limitation for sure. Please note, a packraft generally has a thight fit!
Indications you may want thigh straps are: playboating (riding waves and holes), white water higher than grad III, top heavy loads in whitewater higher than class II.