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.