You can solve all three of those with Wolfram Alpha too, with no need for apps. Seems like Google knows how to solve the last two, but not the first one. Perhaps with some different syntax it would be possible too.
What I like about this app is that there's context and history. I use Spotlight all the time for these conversions, but there's no way for me to keep the last calculations around for reference or reuse; I have to put stuff in a text editor for that, which is a lot more awkward.