I was recently looking at the nikon sg-3ir to cover the flash on my nikon d3300 so I can use it as a flash trigger. However for what they are, the sg-3ir units are quite expensive and I was wondering why nobody had just attached a piece of infrared filter to their camera? Also the build quality of these units is quite poor as some people on Amazon have mentioned. I am also aware that these are becoming quite rare now - at least in the U.K
Actually, some people have just used exposed/developed film as a filter, but in these digital days, that stuff's harder to come by. Infrared filter gels are probably as or more expensive than the device you're looking at.
But generally speaking, most people never need this device for two reasons.
First, light from the pop-up flash rarely registers in off-camera flash photography, because the optical master/commander signal is done at a low power level. Because of the inverse square law, used at normal subject distances, this light tends not to register much if at all in the final image, compared to the main flash burst. And if you're shooting at macro distances, then you're more likely to be using a ring-light setup or some other on-camera combination of flash.
Secondly, optical systems have weaknesses. They require line of sight and low ambient light levels to be really reliable. They work great in studio conditions; not so great out of doors in bright sunlight without bounce surfaces around. For these reasons, most off-camera flash shooters use radio triggers. And you can choose to have a "headless" (i.e., a unit with no flash lamp) unit as your on-camera transmitter. This won't emit any light at all and doesn't use the pop-up flash, so eliminates the need to make the master/commander signal non-visible.
It passes IR and filters out the visible portion of the pre-flash sequence. The IR is required to trigger off-camera flash units. The visible pre-flash can cause humans to blink and I suppose cause birds to fly away.