re: Turning on your hazard lights in rain-what does this accomplish?

I'll go with the hazards if I'm driving 40+ MPH below the speed limit and I can't see the car behind me (could be fog, rain, whatever). Once a car comes up and it's clear they see me and are in tune with the fact that traffic has slowed to a crawl I'll shut them off.

I don't think the law is that clear on this being illegal

"Flashing lights are prohibited except on authorized emergency vehicles, school buses, or on any vehicle as a means of indicating a right or left turn, or the presence of a vehicular traffic hazard requiring unusual care in approaching, overtaking or passing"

I would consider driving substantially below the speed limits to be a hazard requiring unusual care, even if the initiating event (rain) isn't all that unusual.

There is no "prescribed" safe speed for a torrential downpour, it's honestly left up to individual drivers judgment (which, judging by my LA auto insurance rates, isn't that good). Some drivers may feel safe at 50-60MPH and some may only feel safe at 10-15 MPH. I would prefer those that only feel safe at 10-15 MPH during a rainstorm to just get off the road - but that isn't my call and there isn't much I can do other than slow down and get in line behind them.

If I'm driving 10MPH due to traffic flow in a situation where I would honestly feel comfortable driving 40-50MPH, I do worry about getting rear ended. I think the hazards help alert the ridiculously slow movement of cars to those behind me.