What's the purpose of a tactile switch with GND pin?

ESD from fingers is diverted to shield for 15kV test.

Confirmed in spec

• Ground terminal available to protect against static electricity.
• Beware path inductance 1nH / mm (typ) and avalanche static discharge rise time can cause ground shift relative to IC unless over PCB ground plane or further protection of signals and Gnd to IC.

Anecdotal

My 3rd employer in early 80's, Burrough's had > 100 factories including a personal computer factory in Flemington NJ using a design from a company Convergent Technologies(CT) which they purchased in San Jose. They performed Design Validation tests (DVT) ( with Corporate EMI Standards which I assisted in improving, well before IEC similar standards were created) including ESD testing on keyboards. The protroding LED lens caused EOS/ESD failures at 5k~15kV by creepage around LED lens to traces under the surface. By analysis they were able to improve the design to withstand 15kV ESD tests no interruption. Air gaps are ~1~3kV/mm can breakdown on a clean surface. This was a 5mm LED with 5mm exposure to conductors under the the plastic case. The gap to increased to signal with impedance analysis, and increased to ground plane by greater recessing and ground plane improvement.