Each major iOS SDK release includes a full simulator runtime image weighing 6GB to 10GB. Over multiple years of development, your Mac accumulates abandoned runtimes for older iOS and watchOS versions that you no longer test.
Located in /Library/Developer/CoreSimulator/Profiles/Runtimes and ~/Library/Developer/CoreSimulator/Devices, these obsolete simulators take massive amounts of disk space.
XrayMac's Developer module surfaces installed simulator runtimes with their OS versions, letting you delete obsolete device runtimes safely with one click.