Why collaborative robots needed their own standard
ISO 10218 was built around the traditional model of industrial robots as machines that operate in space excluded from people. Collaborative robots, cobots, break that assumption on purpose, they're designed to share a workspace with a human, sometimes working on the same task at the same time. That changes the entire safety question from 'how do we keep the robot and human apart' to 'how do we make it acceptable for the robot and human to actually be in contact, or near-contact, some of the time.' ISO/TS 15066 exists as a technical specification that extends the ideas in ISO 10218 specifically to that collaborative case, addressing hazards and controls that only become relevant once physical separation is no longer the default safety strategy.
Conceptually, ISO/TS 15066 deals with several distinct collaborative operating modes, ranging from a robot that simply monitors for a human's presence and slows down accordingly, to genuine hand-guided operation where a person physically moves the robot, to power and force limited operation where the robot itself is engineered so that any contact it makes with a person stays within levels that are unlikely to cause injury. Each of those modes carries a different safety burden and a different set of engineering requirements, and choosing among them is itself a real design decision shaped by what the application actually needs.