the simple past "X-ed," as compared to the progressive "was X-ing". Compare "I wrote the letters this morning" (i.e. finished writing the letters: an action completed) and "I was writing the letters this morning" (the letters may still be unfinished).

In describing longer time periods, English needs context to maintain the distinction between the habitual ("I called him often in the past" - a habit that has no point of completion) and perfective ("I called him once" - an action completed), although the construct "used to" marks both