רַ֗ק לְמַ֙עַן֙ דַּ֚עַת דֹּר֣וֹת בְּנֵֽי־יִשְׂרָאֵ֔ל לְלַמְּדָ֖ם מִלְחָמָ֑ה רַ֥ק אֲשֶׁר־לְפָנִ֖ים לֹ֥א יְדָעֽוּם׃
only that the generations of the children of Israel might know, to teach them war, at the least such as beforetime knew nothing thereof;
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