רַ֗ק לְמַ֙עַן֙ דַּ֚עַת דֹּר֣וֹת בְּנֵֽי־יִשְׂרָאֵ֔ל לְלַמְּדָ֖ם מִלְחָמָ֑ה רַ֥ק אֲשֶׁר־לְפָנִ֖ים לֹ֥א יְדָעֽוּם׃
이스라엘 자손의 세대 중에 아직 전쟁을 알지 못하는 자에게 그것을 가르쳐 알게 하려하사 남겨두신 열국은
Rashi on Judges
Only so that they should know. For this alone, He permitted these nations to remain, so that future generations of the Bnei Yisroel might grasp the consequence of iniquity—for it was now necessary for them to undergo martial training.
Metzudat David on Judges
ONLY SO THAT THEY SHOULD KNOW: It wants to teach that only because of this they will become aware of the wonders of the miracles that were done for them since then