This is a huge saddle bag with the capacity of many rack trunks (great for road bikes without rear racks). You can pack a picnic and extra clothes while the 3-point attachment and hardwood internal support keep the load stable. You can also use the outer lashing rings to attach a jacket or your walking shoes.
This a traditional design that works best with your saddle mounted in a forward or neutral position on your seatpost. If your saddle is pushed all the way back on the seatpost, you may have some interference with your upper leg. There are various solutions to reduce this overlap including using a small wood spacer between the seatpost and bag to reduce overlap.