You hand over a big, multi-part task in one long prompt. The agent takes it all on, runs straight through, and returns one large block — and now you have to read the whole thing to find where it went wrong, or which part dragged another off course. The context got stuffed with everything at once, so each part came out blander than it would have alone.
A big task is rarely one task — it's a few tasks stuck together. Break it into pieces with their own goals and each one needs only its slice of context, and you get to check each piece before it can drag the whole block off course. This is orchestration even when you only have one agent: split it to see it clearly, not to run it faster.
✕ One block
✓ A few clean slices
Paste it when the task is big enough that you can feel "it's made of a few parts." The trick is making it state the split first and wait for your nod — because the likeliest failure isn't inside any one slice, it's at the seams between them: what belongs to which, and whether one slice needs another's result.