It took me a long time to realise the point of learning 'phonics' at school (they were called phonograms in Australia).
While I can see why writing and reading without speaking makes phonics look useless, for children, all of their language is initially communicated through sound. Putting a bunch of symbols in front of them and saying that it has meaning isn't going to help them - but showing that the symbols roughly correspond to sounds that they already know have meaning is a pretty important step.
This doesn't apply to all written languages - logographic systems obviously don't have a correspondence between sounds and written symbols - but for alphabetic systems and the like it's quite a good system.
Of course, English uses 26 letter to represent 44 or so sounds, so as far as learning to read and write via phonics goes, English is one of the worst examples there is.