A launch checklist grows to fifty items in no particular order, and nobody can tell from it whether the launch is on track. Items from different workstreams are mixed together, the important ones look the same as the trivial ones, and the list says you are “not done” until suddenly you are.
A tree fixes that. Group the work into a few workstreams, break each down only as far as you need, and track partial progress so the project percentage tells you the truth.