why do we use IR filters for night flight

I don't know about IR searchlights, but IR vision systems are being tested for airliner use because IR transmits through fog and haze better than visible light. The IR vision systems I am familiar with are PASSIVE ones, FLIRS, that use the IR signature emitted by virtue of the temperature of the object. One can make a visual approach to a runway that is fogged in and invisible to human vision. If the fog is heavy enough, even the thermal IR cannot penetrate, but lots of the time the runway is invisible in visible light but visible in LWIR.

Unless the laws of physics have changed since I worked on FLIRs, fog and rain are two of their worst enemies...a radar would perform the task you describe FAR better.

I could see a civil system employing a combination of IR illuminators and NVDs - to allow personel in the control tower to visually spot traffic both in the air and on the ground at night within the ATA. That would be useful. Plus the IR searchlights wouldn't interfere wih the flight crew's vision.