Walking boots are made with a specific environment in mind. Characteristics such as sole stiffness or ankle height will vary dependiing on the terrain and conditions you will be heading into.
If, for example, you intend to stick to well-trodden and predictable paths, 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 can aid your comfort on longer journeys.
If, however, you intend to be moving over uneven or very steep ground then you will need a more rigid boot with good ankle support. Stiffer soles will provide a more stable platform to stand on, and a higher cut will support your ankles.
Of course, walking encompasses many different landscapes and the majority of people will be somewhere in the middle of this scale, so err on the side of caution to make sure you're not caught out.