Why do I need to estimate my taxes due for an extension? Wasn't TurboTax collecting this along the way?

[HelenaC]

Yes, however, the reason you are filing an extension is because you don't have all your tax documents. Tax documents could be in the form of missing W-2's, 1099's, K-1's, etc. Those tax forms and undeclared tax deductions may calculate a new tax liability so that is why you manually have to enter an amount.