Fourier analysis

[hanson]

Hi all.
I am learning a numerical method that involves Fourier transform.
As far as I know, I think Fourier transform is tool to find the frequency spectrum of a signal.
And the usual form shall be
"Integrate from negtive infinity to positive infinity, f(x)*exp(i*w*x)dx"

However, when i search this in wikipedia, it is defined with (w-wo), where wo is the central frequency.

http://en.wikipedia.org/wiki/Split-step_method

I don't really know what this definition is doing and what changes it would bring.

Why is it necessary to do so?

Please help.