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 have no idea what would trigger the collapse, but it's extremely annoying to browse this way. Especially on submissions with a lot of comments, you have to go through a huge list of worthless comments to find the next "good comment".
Either a thread collapse or a "skip to next comment on the same indentation level as this" would solve that problem.