it really depends (like in English) for exaple: "jag har arbetat" literally means:" I worked" while "jag har arbetat" literally means: "I have worked" as you see "har" gives your counterpart the general idea that you have done sth, on the other hand, if you not use "har" you`re not say that you have done sth.

So you use perfektum: when an action is "unfinished" (you are still "doing" it) or when time is irrelevant. "Jag har bott i Stockholm" = Ive lived in Stockholm preteritum: when an action is finished, and time is important. "Jag bodde i stockholm förra sommaren"