Why does Ibn al-Jawzi cite weak hadiths in his writings?

The scholar mentions weak hadiths for reasons including: the permissibility of acting upon them in matters of virtuous deeds, or their authenticity according to him while being weak according to others, or strengthening a weak hadith with another, or because they serve as a corroboration for the meaning of an authentic hadith. The scholar is a mujtahid (one who strives to deduce Islamic law), and he is rewarded for his correct conclusions or his errors.