Each surface has a job. The side panel is a full chat that already knows what page you are on. The capture editor opens after a screenshot and handles annotation, cropping, redaction, and sending the capture to a model. The command palette is the fastest path to any action, and every action is also reachable by pointer.
Use the side panel when you have questions about what you are reading, the editor when a screenshot needs work before you use or share it, and the command palette whenever you know the action you want. Open the palette with its shortcut and type the action name.