Without zip top bags my suitcase and backpack would be a disaster. I use them in all sizes to organize and keep things from getting lost in the shuffle of bags from place to place. You know what I mean. Remember that old gorilla baggage handler commercial for American Tourister? We start out packing so neatly and everything shifts. Not only does it shift but it settles as well just from the weight of the clothes or the other bags on top of yours. Here's the way I use those handy dandy helpers.
The 3-1-1 Bag - Most of us are now familiar with this bag. Liquids you intend to carry-on must only be 3 ounces put together into 1 quart sized clear zip top bag and you are allowed only 1 such bag. I don't normally carry toiletries in a carry-on but I do stick my lipstick, lip gloss, hand cream, sanitizer, etc. in one bag--even my chewing gum (unchewed of course).
Medications - Both my husband and I count out our prescription meds that we will need for the trip plus an extra few days in case we get delayed in flights or weather conditions on the road. We put the first week into our pill holders and then the rest in a small plastic snack bag that zips. All of that goes into a larger quart sized zip top bag and I can easily pull it out of my backpack in transit if I need them.
Power cords - Ever get to your destination and have to untangle the power cords from each other or items in your suitcase? If you put them in a zip top bag there will be less hassle because they won't move around as much. If you are so inclined, you could even get a separate bag for each power cord.
Liquids - All right. Yes. My natural color is now gray but I'll never reveal it as long as I can keep coloring the roots. I do not want those liquids floating around my suitcase though. Even though it's not a liquid, I also put my hair spray, spray-on sunblock, or any other aerosol into a zip top bag. I double check to be sure they are zipped tight.
Delicate items - Okay, now this is getting into the area of TMI (Too Much Information) but I personally hate the idea of someone doing a security check of my suitcase and rifling through my underwear. All of that gets put into a zip top bag so that simply by feeling the outside of it and looking through the clear plastic, an inspector can tell there's nothing hidden inside that would be a security risk. Sure, it's just my quirkiness but I feel better doing that.
Vacuum pack - If you have a clothing item that is made from material that is slippery, silk for instance, you might want to put it into a little larger zip top bag and work the air out of it before you seal it. This will help it to lay flatter in the suitcase and not slip between other things to crease and wrinkle.
That takes care of all the areas I can think of. If you have some more tricks for packing with zip top bags, please share them in a comment below.