A whiteboard is usually a separate subscription, which means the diagram explaining a project lives somewhere other than the project. Six months later the work is in one tool and the picture of how it fits together is in another, one version behind.
Planner.day boards sit in the workspace with everything else.
Realtime, genuinely
Several people can edit one canvas at the same time, each with a visible cursor. There is no locking, no save button and no merge step. This is the part that decides whether a whiteboard gets used in a live conversation or only after one.
Shapes
The shape library covers the usual flowchart and box-and-arrow vocabulary, and includes cloud architecture icons, so a systems diagram looks like a systems diagram rather than an approximation built from rectangles.
Diagrams from a prompt
Describe what you want in writing and the board generates the diagram. This is most useful for the first pass, which is the part people avoid because a blank canvas is harder than an edit. Generate a structure, then move things until it is right.
Sharing outside the workspace
A board can be published through a public link, which is how you show an architecture sketch to a contractor or a client without provisioning them a seat. The link can be revoked.
Where they fit
Boards belong to the workspace, so the diagram for a project sits with the project. That is the whole argument for having them here rather than in a dedicated tool: not that the canvas is better, but that it is in the same place as the work.