The problem
GTM planning is strategic and messy in the best way. You're answering: who are we selling to, how do we reach them, what do we say, what do we charge, and in what order do we do it? These decisions interact. Price affects positioning. Positioning affects channel. Channel affects sequencing.
A whiteboard holds all of these in tension simultaneously. You can draw connections, challenge assumptions in real time, and see the strategy take shape spatially in a way no linear document allows.
Then you need a GTM document to execute from. And writing it from memory, a week after the planning session, is not the same as capturing it from the board.