The sidebar menu gives you more space to organise your navigation. The current system of it being in the header means a lot has to be hidden to make it fit, which isn't the best onboarding experience nor does it help highlight important areas of the community.
The thought behind the node menu item was to make it easier to add specific forums (or gallery albums, etc) so you can better design your community. You may want to highlight a few key areas (we might choose to feature Community Insider, Support, Feedback) making those areas easy to reach wherever you are and to help new visitors to your community discover what is important. Likewise, you might wish to enable fluid view and list your individual forums in the sidebar so you have the best of both worlds, a feed view index and then easy access to specific forums.
The theme for Invision Community 5 is optimising the minutes you spend on a community to make them more impactful. We are considering the various lifecycles of a topic. The initial burst of energy that crackles with noise and distractions and also in the future where future visitors come to it cold and want to extract key information quickly.