After a meeting, memory starts editing the plan.
A short decision note stops that drift because it gives the group 1 shared reference point.
Writing the agreed actions right away means people return to the same details instead of rebuilding the discussion from memory.
Without that note, different versions of the decision can become different plans, and the mismatch shows up later as duplicated work or frustration.
The note does not need ceremony. It just turns a spoken agreement into the thing everyone can point at.