One of the best tricks we have found when designing our living space is proportion. Most of our truly great furniture finds have come from random places while we were out and about... not from when we have "left the house determined to buy a sofa."amazing French Provincial couch that you think would be the best thing since sliced bread in your living room. You drag it home, wrangle it through the door frame, scoot everything out of the way... only to find out that it's really 3 inches taller than it should be, or 4 inches deeper than it needs to be, or one shade greener than it can be to work with your room. So you're stuck selling it on Craigslist, or in the paper, before you can find another one. And then the whole process starts all over again.
By having this on hand, be it in the glovebox of your car, your purse, man bag... whatever the case may be, it can save you a ridiculous amount of headache and second guessing.
Don't worry if they are hand drawn, they are only reminders of what's back at home, although if you want to wow the sales associates, feel free to Photoshop until your heart's content.
You can also laminate your layouts so they are spill resistant, although a note of caution, the color scanners at the hardware stores, are unable to mix paint accurately if they have a coating on them. If you would like that extra layer of protection, consider putting your paint chip on the ring lose so it's able to be used when needs be.
All of these things are nice to have on hand, so when you find "the perfect piece" you will know before you get it home if it will really work in your space.