When user first logs in, they would see the left hand menu on the left of the page which is opened by default. They can choose to keep or unpin the menu by clicking the hamburger icon in the top left of the screen. When unpinned, the site page widens and fills the page including where the left hand menu is, this can allow for more to be seen on the page, this is especially good if your using a smaller screen and want to fit more on the page (Phone Screen). That was one benefit to the left hand menu being closed, a benefit for being open is changing from form to form, report to report and dashboard to dashboard is much easier with it open. There are also more benefits below
It also gives the option to create new records at the top in the menu, you can also see how many records you have above that and go back to the app homepage by clicking on the app Icon.