What is a reason that we need to be able to use the quadratic formula rather than solving all quadratic equations by graphing?

Here are some examples of how the Quadratic Formula works:

Solve x2 + 3x - 4 = 0

This quadratic happens to factor:

x2 + 3x - 4 = (x + 4) (x - 1) = 0

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Then, as expected, the solution is x = - 4, x = 1.