There are probably other reasons people use them, but a few reasons why planter boxes can be the right choice:
(1) If you live in an area with very poor soil conditions, it may take years and lots of hard work to get your soil usable for the things you want to plant. Or you can grab planting soil that meets your requirements upfront and dump it into some planter boxes.
(2) They look nice, and some people are going for that look in their garden.
(3) Not needing to kneel so much, which can be a game changer for people with eg knee problems.
4) Not having to actually dig to plant. The work involved in buying soil and dumping in a planter box usually pales in comparison to digging a hole a couple-to-few feet deep, wide, and long.
(1) If you live in an area with very poor soil conditions, it may take years and lots of hard work to get your soil usable for the things you want to plant. Or you can grab planting soil that meets your requirements upfront and dump it into some planter boxes.
(2) They look nice, and some people are going for that look in their garden.
(3) Not needing to kneel so much, which can be a game changer for people with eg knee problems.