Can I say "why do you write them for?" instead?

Native speaker’s answer
Rebecca
No, it would sound unnatural if you were to say "why do you write them for?" here. This is because the expression "what do you write them for" is similar to asking "why do you write them?". The word "for" is not necessary after "why", but when used after "what", it often indicates "why". Ex: What do you use these shoes for? Ex: What are these pillows for?