The word 顺便 is formed by combining the character 顺, which conveys the idea of "following along" or "going with the natural flow" of a situation or journey, with the character 便, which carries the meaning of "convenience" or "ease". Together, they conceptually merge into the notion of "following the path of convenience", where one action is naturally and easily attached to another, like picking something up because you're already going that way. It's a logical pairing where the first character sets the condition (going along a certain course) and the second character provides the manner (doing so with added convenience).