Because they are iconic. You can't have Cinderella without the glass slipper; much like you can't have Sleeping Beauty without a spinning wheel.
Unfortunately, they loose meaning when movies want to take a more modern approach to Cinderella's meeting with the prince. the slipper was the device used because the prince had no idea who Cinderella was and he had no other way to identify her in a vast kingdom of women. If he knows what she looks like as a commoner he can find her more easily and it defeats the purpose. Of course, they didn't go this route in the film and, I just think it makes the story slightly convoluted when they have to perform summersaults to get the film to align with the slipper narrative. That being said, I adore this film and it is what it is.
My favorite use of the slipper was in 'Ever After' where Henry knew the slipper was Danielle's the whole time and they played off the slipper fitting as a joke between them or as a 'you belong in these shoes now' meaning.