This is a ease of use and safety-oriented decision: magnetic visors allow for interchangeability, so one rider can easily swap between visors based on conditions (think of switching lenses in a ski goggle). The magnets keep the visor in place securely but in the event of a crash, will fly away from your face so they don't become a hazard. Fixed or pivoting visors will often become lodged in the rider's face upon impact, doing more damage than they do protection. A magnetic attachment prevents this from happening.