You're right! It's a good job there are no words in English where one spelling encompasses many meanings, of which some could even be contradictory, which would have to be inferred from context. That would be so bad!
But for good or bad, it is part of the language. People used to it consider replacing it with 'ss' as ugly. Because of custom, and also because a double consonant almost always indicates a short vowel, whereas a single 's' indicates a soft s. Only ß actually represents the intended pronunciation.
f course w'd gt by witout it. Bt Englis wld similry srvive vs fewr lttrs, so y nt ablsh 1/2 t lphbt?