One of the first things that they ever taught me in boy scouts was the boy scout motto, "Be Prepared." It's something that I have incorporated into my everyday life, not just boy scouts. Whether it was work, school, or everyday life I always did my best to be prepared. However, no matter how hard I worked towards being ready for anything, there was always something that I would forget. Nine times out of ten this was my rain gear. I can't even remember how many times I used a plastic trash bag as a poncho when I found myself in a downpour with no rain coat.
Rain gear is something that is extremely important to have with you pretty much at all times. You never know when the weatherman is going to be wrong... or right for that matter. Rain gear is especially important if you plan any trip for the outdoors. Not only will it keep you dry but you can use rain gear for many other purposes as well.
Ponchos
Plastic ponchos are a fantastic thing to keep in your backpack or car. They are quick, easy, and pack very nicely. They can also have many other purposes. There tons of uses for a bit of plastic sheeting. They're relatively durable and good for storing your items in if you don't want them to get wet. You can also make a quick, waterproof shelter out of one. I'm not saying it would be a regular Holiday Inn but it would do in a pinch. I myself have used ponchos a number of times to waterproof a shelter.
Rain coats
Rain coats don't have a lot of extra purposes but if you have some ingenuity and resources available you can figure something out. For example, if you tied up the sleeves and the hole for your head you could mack a quick rucksack out of it. You could also use it as a signal flag if you are in a situation where you need help.
Umbrellas
Umbrellas can provide you with a ton of resources for a survival situation if you had one available. You could make snares, traps, hooks, shelter pieces, and tons of other things as well. There are invaluable uses for the metal pieces that make up the structure, the handle as well.