Texanne, they are both strong (enough for what they can hold) as well as uniform in size & that means they both stack easily & are easy to move.
Additionally, they are just the 'right size' for dishes & glassware that normally needs a boatload of newspaper or other packing junk to keep from breaking.
You can easily move a dresser's worth of clothes with 10 boxes & it's easy to move them around.