- It's attention-grabbing. Obvious.
- Some drivers see a motorcycle, but misjudge it's speed (leading to a collision). The same thing happens when motorists try to beat a train to a railroad crossing -- the sizes of motorcycles and trains are different than the size of cars, which makes it more difficult to judge their relative speed. A headlight modulator knocks a driver's brain out of it's "comfort zone" and forces a re-assessment of the situation.
- Most new cars are wired so that their headlights are on all the time, even during the day. Motorcycles also have their headlights wired to be on during the day, so motorcycles now tend to "blend in" with a pack of lighted cars. A modulator helps make them stand out again (hopefully, modulators will not ever be allowed on cars).
I don't have it set to modulate all the time, however, as the flashing in the rearview mirrors can be annoying for the person immediately in front of me. Since I'm constantly scanning side streets and oncoming traffic for people who may become a threat to me, I just flip it on whenever the situation warrants it.
You may be wondering if a flashing light is legal if I'm not an emergency vehicle. A modulator is 50-state legal for civilian motorcycles per the Federal Department of Transportation if it meets certain specifications (which mine does) in regards to flash rate and time of day (it has an 'eye' to sense the light level and automatically disable itself at night). The DOT has done a lot of stupid things, but this was one thing they got right.
And it does get noticed. A couple of times now, I've flicked it on and been satisfied to see a driver's head swivel to follow me as I ride past (they're probably thinking "is that a motor cop?"). The device was fairly expensive, but cheaper than a hospital visit. Plus, it gives some piece of mind, which is -- as they say -- priceless.