Partly that, partly ergonomics, and one reason that no longer applies….
Here we have very clay heavy soil, so it’s easier to go up, and the more viable soil the more densely you can pack your garden beds. With raised beds you can avoid planting row style for more dense plantations since you aren’t walking between the plants compressing the soil.
Raised beds warm up earlier I’ve heard, haven’t tested that one yet, but supposedly you can plant earlier with them too.
Ergonomics should be self explanatory, less bending.
And I sadly had to put my blind dog away last week, so the taller beds were so she couldn’t waltz through them.