I started using a plastic sandwich box over 30 years ago on my backpacking trips. I used it as a dinner bowl and then packed any leftovers from the meal into it for lunch the next day. And though I don't backpack very often anymore, I still find a plastic sandwich box one of my most cherished travel items. Here are three uses for it:
1. When I travel, it helps me keep small items organized all in one place - batteries, laundry soap and clothespins, my mini-flashlight etc.
2. I use one to make and then carry my favorite trail mix. Get, or make, regular trail mix, i.e. nuts and dried fruit. Then melt chocolate chip morsels in a double boiler and stir into the trail mix until it is thoroughly coated. Put it into your plastic sandwich box, stick the box into the freezer, and leave it there until you're ready to go.
3. I often use one as a mixing bowl when I'm traveling. I buy a packet or can of tuna, use packets of mayonnaise, and then mix it all up in my sandwich box.
And of course I still use if for sandwiches! Not only does it protect a sandwich from being crushed, it is easy to clean and infinitely reusable, thereby cutting down on plastic waste. I also use it to protect other crushable food, such as grapes. And I use it to carry foods like cheese - cheese does not actually need to be kept refrigerated to stay good, but the oil will seperate out, especually in warmer climates, and can get rather messy - not a problem if it's in a sandwich box.
I strongly prefer not to eat out in a restaurant 3 times a day when I'm traveling-it takes too long and is too high in calories. So often I stop in a local market and buy fruit, bread and cheese or a sandwich. With my trusty sandwich box in my pack, I know I'll have a great lunch at exactly the right time in the right place.