The schwa is the most common (non) sound in our language.
I posted a 3 part video back in 2018 modelling ‘pure’ sounds:
The etymology of ‘schwa’:
From Michael Rosen, this is why the schwa makes it difficult to blend (reading) or spell (writing):
For children who are effortless decoders and encoders (around 10 in every class) it doesn’t really have an impact – for the rest it does!
The schwa is not just connected to phonics and the making of ‘pure’ sounds but lurks in many words, making spelling so difficult.
This is why the teaching of affixes as units is so important:there’s a schwa in ‘ment’ and many other suffixes:
See this extract from Joe Moran’s excellent ‘First You Write a Sentence’: