Heavy rain can soak all of your gear. There are some things that you don't ever want to get wet, like your camera. I've learned to protect those things with string bags, which are light and I always have a few with me.
However, it would be nice to protect my whole backpack from rain. A long, broad poncho is great, but I lost my good one and the next I bought was quite heavy and doesn't cover the whole backpack. When traveling abroad I have left it at home because of the size and weight.
Now I'm looking for concept to improvise some protection for my backpack during a trip. Garbage bags are good to hide a backpack inside and wait for rain to stop, but the problem with them is you can put the backpack inside the garbage bag, but then you can't put the backpack back on.