The rate moves with the mix of countries the profit is earned in, one-off settlements, tax credits and share-based compensation deductions. A quarter that beats on eps purely because the tax rate fell is not the same as one that beats on demand.
Most companies guide to an expected full-year rate. A change in that guide is a mechanical change to forecast EPS across every model on the street.
Example: Northwind Tools books $33M of tax on $132M of pre-tax income, a 25.0% effective rate. If a research credit cut it to 21%, net income would be $104M instead of $99M and EPS would rise about 5% with no operating change.
Related: income-tax-expense, pre-tax-income, net-income, eps, guidance