It's mainly for CERT (I just joined last year). Generally viruses/airborne and even transmitted by fluid. Seems like the N95 masks are fine and a respirator might be a bit overkill then.
If we get a patient with a productive cough, we put a surgical mask on them to catch the big stuff and N95s on ourselves to catch any airborne droplets, so I'd say you are on the right track with the N95s.
You aren't going to stop everything with one, but by reducing the amount of a virus/bacteria you inhale, you reduce the chance of contracting it.
I was thinking about the usefulness of particulate filtering in certain circumstances. In a "dust bowl" type scenario, or a building collapse in an urban area (think 9/11 aftermath), the dust that is in the air would be a big issue for people, but I still think a bunch of N95 masks would be better than a half face respirator, even with a few spare cartridges. I live in the Northeast in a rural area, so neither of these is really likely for me to encounter.