Is there any reason to make and use a barrel planter instead of a flower box?

[Freddy]

Not only do you need less soil for flower boxes, they also have three slots instead of one, and they both are XL.

[wolfdyke]

In terms of the reasons you're probably asking, nothing really afaik? But in a broader hypothetical sense...
Sometimes you only have the recipe for one and want planters Now. Other than that, material availability, maybe they look better to someone in the right places, and might optimize space usage in fringe scenarios.
So, nothing especially major, same reason as any other better vs. worse craftables- why make a hammock when you can make a bed that doesn't kill your mental health? Because sometimes you need something for day one of a save when you have 10 oc, what you could dig out of the trash, sparse recipes, and hope. Or you think that specific thing looks nice/funny in a fancy screenshot.