Usually a marked term has an unmarked opposite, such as "happy" (unmarked) vs. "unhappy" (marked), "old" (unmarked) vs. "young" (marked) and "big" (unmarked) vs. "little" (marked). So you don't generally have unmarked and marked terms meaning exactly the same thing, from what I can tell.
For male/female distinctions (at least in English), unaffixed male terms are often unmarked, while affixed female terms are often marked, as can be see from "man"/"woman" and "lion"/"lioness" distinction, where "man" and "lion" are much more broad and dominant terms than "woman" and "lioness", which are quite specific. Furthermore, singular forms are usually unmarked, while plural forms are often marked.
So usually, marked terms are not simply marked versions of unmarked terms, they have different meanings even when not considering their markedness.
It has been theorised that markedness reduces cognitive complexity.