As-Salaam alaikum,
In his famous work titled'al-Faid ar-Rabbani', Sayyadi Abdulqadir al-Jilani, may Allah be Merciful to him, observed that ''It is not only by saying LA ILAHA ILLALLAH on the tongue, but by the reality in the heart. That is why we have to recite [the Kalimat Shahada] 100-1,000 times, and will burn Shaytan from among the jinn and ins who are doing everything without limits, annahu naaran, it is fire to shayaateen and a light for belieevers.''

Fire can burn fire; a huge fire takes over the smaller fire, which will disappear. So LA-ILAHA ILLALLAH is a big fire that takes Shaytan's fire away and also your sins, and it is light for muwaahidoon. ​The sun is an example for this-- the fire on the sun gives us life. Naar becomes shinning light, and it is fire but it gives you life. Without the sun you cannot live. So that LA-ILAHA ILLALLAH will burn Shaytan and becomes Light for believers.