You text should say: I *want to* say phrase like "I don't know what *___* is needed for" but I don't think it is *a* correct phrase. How *should I* properly say "I don't know the reason that *the* thing is here"? It's hard to gauge context to answer your question, but we would say "I don't know what this is used for" or "I don't know what it is used for", if you see something and you don't know how it works. If something just showed up and you don't know *why* it's there, then you could say "I don't know why it's here" or "I don't know what it's doing here."