Open Tasks/Projects lets you see almost all information about the task in one place. It contains your task note, attachments, subtasks, and the edit panel where you can make changes to your task. Besides these, it also displays your hover buttons which you can use to make quick changes to the task.
When you first open a task, you will only have one column. To add more columns, click on the left or right arrow on the top right, underneath the hover buttons: