Many years ago, I worked with an organization that had a motorcycle ride for charity, and I spoke to a lot of the riders afterwards about why some of them had very loud motorcycles.
They explained that, (at least for some of them), it was for safety reasons because some drivers don't notice motorcycles on the roads, so they feel invisible, and that's dangerous for them. They prefer the bike to be loud so that cars will hear them, and they won't get hit by other drivers who can't see them.
While I respect motorcyclists in general, I have questioned some of their explanations.
Under the logic that since motorcycles are less protected & smaller than cars they need to be loud for car drivers to hear them, the same argument could be made that cars should be loud so that tractor-trailer truck divers can hear them since cars are smaller and stand little chance in a collision with a large truck. Yet, cars aren't made louder so large trucks can hear them.
Conversely, I've had many experiences where I was approached by excessively loud motorcycles while driving and was actually startled & distracted by what sounded like I was suddenly being bombed. Startling drivers (which may cause them to take their eyes off the road, start franticly checking mirrors or even weave out of their lane) like that is far more dangerous than just approaching or passing safely & quietly.
I'm still awaiting an explanation why: the louder the engine, in a residential area, the longer a motorcyclist has to rev it to "warm it up"?
When the reality is it only takes a few seconds for a combustion engine to reach its optimal heat level. Most people who "warm up" cars do so to heat the interiors so they feel more comfortable or defrost the windshield, but even in cold temps, there is no need to "warm up" a car's engine beyond a minute or so. Motorcyclists can't even use the excuse that they are heating the interior or defrosting the windshield, so running and revving the engine is yet another act of attention-seeking inconsideration via disturbing the peace.