Anyone who built software for a while knows that estimating how long something is going to take is hard. It's hard to come up with an unbiased estimate of how long something will take, when fundamentally the work in itself is about solving something.
Breaking larger tasks down effectively removes uncertainty.
My general rule of thumb in planning is that any task that is estimated for longer than 1 day should be broken up.
Longer than one day communicates that the person doing the estimate knows it’s a large task, but not super clear about the details. It also puts a boundary around how long someone waits before trying to re-scope:
A task that was expected to take one week, but ends up going 2x is a slide of a week, but a task that is estimated at one day but takes 3x before re-scope is a loss of 2 days.
You can pick up one or two days, but probably not one or two weeks.