Why does Orwell's appendix on Newspeak matter?

Because it is written in the past tense. The novel ends in defeat - Winston broken, loving Big Brother - and then there is an appendix explaining the principles of Newspeak as a historical subject, describing what the language was and noting that the final adoption had been set for 2050. Something is looking back at the regime from after it.

It also uses standard English throughout and quotes the Declaration of Independence as an example of a passage Newspeak could not translate. A scholarly essay in ordinary English about a dead language is not a document the Party would have produced, which is the whole implication and one Orwell never spelled out.

The appendix is the argument of the book rather than a supplement to it. Newspeak works by removing words: no word for freedom in the political sense means the thought becomes progressively harder to hold, and the goal stated in the text is to make heretical thinking literally impossible for want of vocabulary. Orwell had made the same case two years earlier without the plot, in Politics and the English Language - that vague, dishonest language and vague, dishonest politics sustain each other. Publishers have periodically wanted to cut the appendix, and he refused, including to an American book club that wanted it and the Goldstein chapter removed.