You can touch your credit card at the pump without going inside to pay, send videos as messages through a wide variety of social media apps, and even digital signatures for legal and binding documents are replacing hand-written forms. So, why do schools still teach handwriting?
Fine motor skills
Opposable thumbs are used by a variety of primates and key in humans for manipulation of objects. Try doing daily tasks without using your thumbs. It's not comfortable or easy. The small muscles in the hands control the hand, fingers, and thumbs and assist us with manipulating tools and doing tasks ranging from feeding ourselves to manipulating small objects, such as buttons or zippers, and even assist with managing personal hygiene (i.e. brushing teeth, combing hair, bathing, etc.) Handwriting conditions the muscles of the hands to do these types of tasks.
Speaking is just one form of communication. Connecting speaking and correlating sounds with the corresponding letters is the basis of phonics, the foundation of teaching reading skills. Writing is great phonics practice as kids actively connect sounds with letters. The formation of the letter through writing is a second way of processing the information--registering it in the brain in a different way to "capture" or own language. Working from sound-to-symbol is the alphabetic principle.
The first words children read are often the first ones they write, building self-confidence with reading. The self-confidence is important as children use the writing to begin early understanding of punctuation, grammar, and spelling. A confident writer is a confident reader and one who is willing to embrace the challenge that often comes with developing reading skills. Later, writing about what you read will be a way to demonstrate comprehension and solidify concepts.