I've never had to rely on rainwater collection for drinking water.

But I often have a "dry area" covered with a tarp, rigged to drain off at a couple of points that takes the water out of camp. A tin can or pail gives water that's great for washing, and it's a lot cleaner than the stuff in the stream.

If the water is for drinking, it's worth being a little careful regarding purity. A few considerations come to mind:

- rainwater falling directly from the sky isn't sterile, but normally there isn't anything that will harm you (unless downwind of some heavy duty industry)

- rainwater that filters through a treed canopy has considerably more bio-activity, everything from bird doo to tree fungus; I'd be inclined to bring this to a boil if I had the option, but it's still considerably cleaner, pathogen-wise, than the river

- if I were collecting drinking water off a tarp, a quick swipe with soapy water would add confidence, same as washing your hands before eating

- be careful about using trash bags to collect water; some have antimicrobial or scented gunk sprayed on them

- if collecting close to the ocean, a bit of salt spray could make the water brackish; fine for washing, but not pleasant for drinking

BTW, in winter, a suitable black trashbag in direct sun can generate quite a bit of "free" water from snow. I've used this for drinking with no ill effects (so far).