They made life harder for Nazis.
1} Blew up factories delivering to the Germans.
2} Sabotaged railways (and to a much lesser extent air planes and ships).
3} Spread uncensored information to the general public to boost morale.
4} Helped allied air force personnel shot down over Denmark esape via Sweden.
5} Mainly in the final year they killed approx. 400 informers (suspected of) working for the Germans. This was NOT done as a punishment, but to prevent the Germans from catching members of the resistance.