A common request for the Mac version has been to move Signal to a menu bar icon so that it no longer takes up space in the dock. In the past most of the development focus has been on building up the web interface, so admittedly the Mac interface was put together quickly so that more time could be spent on making the web interface look good and perform well.
I’m happy to say that not only has your request been heard, but done one better - in the upcoming 1.2 release Signal is moving to a System Preference Pane. This allows Signal to run completely in the background with no UI whatsoever unless called upon. As an added bonus Signal will now start when your Mac starts, so it’s always available whenever you need it.