Question

Why might you complete 1040 instead of a 1040EZ

Answer 1

You may complete a 1040 instead of a 1040EZ because a 1040 is the standard IRS form used when completing documents to file annual tax returns. A 1040EZ is used when there is a taxable income of less than $100,000 and no dependents to claim. This form is the easy “EZ” version of the full length 1040 IRS form.

Answer 2

because compared to 1040EZ, 1040 is far more complex.

It's got more data/information that supposed to be filled in.
But on the bright side, it provide more chances for tax deductions if you're using it correctly.