The problem
RACI matrices exist because cross-functional work fails when accountability is assumed rather than assigned. Responsible (does the work), Accountable (owns the outcome), Consulted (provides input), Informed (kept in the loop) — clarifying these four roles for every major task or decision eliminates the 'I thought you were handling that' conversations that derail projects.
Building a RACI on a whiteboard is fast because you can draw the grid, fill in the R/A/C/I assignments collaboratively, and negotiate disagreements in real time. A PM who thinks they're Responsible for a decision and an engineer who also thinks they're Responsible will surface that conflict in the RACI session — where it's a five-minute conversation instead of a two-week project delay.
The problem is documentation. A RACI matrix is only valuable if people reference it during the project. A photo of a whiteboard grid isn't referenced. A clean document in the team's wiki is. Producing that document from the whiteboard takes 30 minutes of careful transcription — and it usually gets done imperfectly, missing some cells or getting R and A swapped.