Why are for $A\frac{G}{?}$ deductions preferable to from $A\frac{G}{?}$ deductions?

For AGI deductions reduce AGI thus increasing deductibility of from AG$/$ deductions based on AGI.

For $A\frac{G}{?}$ deductions cause a dollar for dollar drop in tax liability.

For $A\frac{G}{?}$ deductions are only taken if itemized deduction exceeds standard deduction.