Walking boots are made with a specific environment in mind. This means that depending on the severity of the terrain and the conditions you will be heading into, the characteristics of the boot, such as the stiffness of the sole or the height of the ankle are specifically designed for the demands you will face.
If for example you intend to stick to well-trodden and predictable paths then you can go for a lightweight boot or shoe with a reasonable amount of flexibility. In less challenging terrain flexibility is good as it helps your foot move naturally and on long journeys this can aid your comfort.
If however, you are at the other end of the scale and intend to be moving over uneven, broken or very steep ground then you will need a more rigid boot with good ankle support. If your boots have stiffer soles, you’ll have a more stable platform to stand on, and in essence a higher cut boot behaves like scaffolding, supporting your ankles.
Of course, walking encompasses many different landscapes and the majority of people will be somewhere in the middle of this scale, but you get the idea; the further you’re likely to stray from even, flat land, the more sturdy and supportive your boots need to be.