Because of its simplicity, I use it much more than I use the actual HN website.
I've even emailed chpwn (Grant Paul) the creator of the app with ideas for making it better and he was very quick to respond and sincerely consider my suggestions. In the end he chose not to use them, as he feared the UI implementation would not be intuitive.
I suggested swiping horizontally on a comment would collapse the children.
Either a thread collapse or a "skip to next comment on the same indentation level as this" would solve that problem.