The shoulder belt is used to comply with FAA's 16G rule. For most seats on the airplane, the seats in front of you or the bulkhead would help absorb the impact. However, for larger pod-like seats, where you can't reach the "wall" in front of you, the options to meet the rule are either installing an airbag system (those bulky, fanny pack-like bag attached to a seat belt), or use a shoulder belt restraint to comply with the requirement.
This is the same reason why the former IPTE first class seats had a shoulder belt as well.