Let us first divide Arabic letters into two catogaries: solar and lunar شمسية وقمرية. Each catogary has 14 letters and the total is 28. Now if we attach قَمر to the definite article ال it is written القَمر. read more
Or the word hell as helil. So having two lams without a shedda would make it pronounced as Alalah. Like شلل which means Paralysis and pronounced shelel. So adding a shadda breaks that ambiguity. Also, the word Allah isn't the most linguistically standard word in Arabic. read more