clarification on FFT via DSP

[robismyname]

I would like to know something about DSP's in general. Suppose I am using a DSP to do a FFT. Once I get the frequency domain representation of signal what are some practical things I can do with the frequency domain signal I just generated? What is most commonly done with the frequency domain signal?

Also why is it better to change the signal from the time domain to the frequency domain?

I prefer a practical answer.

 

[RCinFLA]

FFT's have a great number of uses.

Generate quadrature signals from single input for Hilbert transform AM detector.
Rolling butterfly interpolation of ADC sampled waveforms to fill in points between ADC sample points. Digital oscilloscope use this to reconstruct sampled waveform (known at sinX/X interpolation).

Just to name a couple of examples.