Commercial window-cleaning robots like the Hobot 388 work by suction — a vacuum pump sticks the unit to the glass while a brushed pad scrubs. They cost $250-450 and have a known failure mode: if the vacuum loses power for any reason (a tripped breaker, the cord catches on something), the robot falls. Tethered safety lines mitigate this but are awkward indoors and not safe at any height outdoors.
The magnetic approach trades the vacuum pump for a passive magnetic coupling: one half of the robot is on the inside of the glass, the other on the outside, held together by neodymium magnets. The coupling force is set by the magnet stack, not by an active system, so a power loss doesn't drop the unit — both halves stay on the glass and you can reset them by hand. Works on glass between 2mm and 28mm thick depending on the magnet size.
The control electronics live in the inside half (no weather sealing needed). The outside half is just two magnet pads, two motorized wheels with rubber tread, a microfiber pad and a tiny water-mist pump fed by silicone tubing. Hall-effect sensors at the four corners of the inside unit detect the edge of the glass (where the magnetic field drops because there's no steel-backed framing) and trigger a serpentine reversal. The whole thing covers a 1m² window in about 8 minutes.