Dandasana may look like a relaxed seated posture, but in fact, your whole body has to engage to hold the pose. It's quite challenging when you put in the work. You need to ground the legs into the floor, engage your thighs, flex your feet, roll the shoulders back, broaden the chest, whilst pressing the hands into the floor or blocks. To make it even more intense, you can lift your arms up. The longer you stay, the more you experience, which is why the pose is challenging.
It's a brilliant pose to build up core strength in a very safe way; great for beginners and advanced practitioners alike. You can use it as the starting pose before and between any other seated poses. Tadasana / Mountain Pose is the blueprint for the standing poses. Dandasana / Staff pose is the blueprint for the seated poses.
You may need to sit up on a blanket or blocks if you cannot sit with a straight back in this pose. Also it can be useful to place your hands on blocks if you find it difficult to sit up straight and have your palms on the floor. Bring this pose into your practice and notice how your posture, core strength and leg strength improves!