This pose, which can be difficult, builds strength in the torso muscles which supports the weight of your spine, rib cage, and head against the pull of gravity. Deeper practice of this asana over time develops our arm and shoulder muscles helping us not to slump, increasing our chest opening and providing an increased capacity to breathe deeper. The increased range of motion you will develop over time in your hips will provide expanded range of motion for the hip joints providing increased mobility in the hip joints.
Option A is a more gentle version and is recommended for those who have less flexibility in the hamstrings and hip adductors and lower back. Option B is a more advanced expression and requires greater flexibility in the hamstrings, and hip adductors. Reaching the advanced version of Trikonasana takes time and practice to expand and lengthen the hamstrings, hip adductors and lower back area.