Why is there any need for Satan in Islam when Allah is the source of evil in Islam?

" That both good and evil are from Allah is clearly stated in the Qur'an al-karim and hadith-i sharifs. All Islamic scholars to date, without exception, said that good and evil are from Allah. The belief that evil is not from Allah is the view held by the followers of Christianity, the Mu'tazilah, and some heretical sects. No Ahl as-Sunnah scholar stated that evil was not from Allah because no scholar can say anything against the Qur'an al-karim and hadith-i sharifs. Humans cannot create their own qadar. As a matter of fact, the Qur'an al-karim says (what means):

(If any good reaches them, they say, “This is from Allah,” but if any evil befalls them, they say, “This happened because of you.” Say: “All things are from Allah.” What is wrong with these people that they do not understand any word?) An-Nisa' 78 "