When you pack your clothes into packing cubes, you avoid embarassing underwear explosions at TSA when you're asked to open your bag, you have an easier time keeping your dirty and clean clothes separate at the end of a trip, and your clothes are less likely to pick up wrinkles en route, because they'll be more tightly packed and less likely to get balled up at the bottom of your suitcase.
But the best case for packing cubes? Packing more stuff! The zip-up sleeves act as an extra layer of compression for your clothes before you even stuff them into your suitcase, and I can easily pack for an entire week in a carry-on when I put everything into packing cubes. And of course, since these are from eBags, they're also backed up by a limited lifetime warranty if they ever tear or the zippers ever fail (though I've never had any issues with mine).
Normally priced at $30 for a pack of three, you can clip the 20% coupon to get them for $24 right now on Amazon, in a variety of colors.
eBags packing Cubes 3-Pack | $24 | Normally $30 | Clip the 20% coupon