Why did he say that Hashem illuminates his lamp?
Rashi: This was when David fought at night against the troop of Amalek that came against Tziklag - "va'Yakem David meha'Neshef v'Ad ha'Erev l'Macharasam" (Shmuel I, 30:17). Radak (30) - Hashem sent lightning to illuminate for David.
Radak: The affliction is darkness, and the salvation is light. Therefore, it says Yagi'ah Chashki.
Malbim: Man, based on his physical nature, is not prepared to bask in Divine light and know [deep] truths. His intellect is prepared for the light. The light of the intellect is compared to a lamp that illuminates the darkness of his Nefesh - "Ner Hashem Nishmas Adam" (Mishlei 20:27). This lamp does not illuminate by itself, only via Hashem's light. Intellect alone would not attain the true light via investigation.
He already said that Hashem illuminates his lamp. What does "Yagi'ah Chashki" add?
Malbim: Darkness and thick cloud surround (physical powers) surround the light of one's intellect. It cannot receive Hashem's light, until Hashem illuminates the darkness. The lamp is a parable for making known true Emunah and Nevu'ah to influence on man's intellect. Illumination of the darkness is a parable for Hashem's Mitzvos, which remove the clouds of physicality via Kedushah and clarity.
