I was recently on a Delta flight to Jacksonville, Florida, and it reminded me exactly why underseat carry-on luggage matters. I was traveling with a small rolling carry-on that held my laptop, medications, and other valuables. On both flights, the plane was small and most passengers were required to gate-check their carry-on bags. That meant I had to hurriedly unpack my suitcase at the gate—stuffing essentials into my purse and carrying the rest in my hands. To avoid this hassle in future travels, I plan to maximize space in your carryon bag by using packing cubes and compression bags....