Fourier transform before calculating the phase
The discrete Fourier transform (DFT) does by definition transform your image into the frequency domain, mapping it to a matrix of complex numbers.

Again, by definition, a complex number has a phase and a magnitude component. 

Thus, you don't "need to perform FT before calculating the phase of an image", you get the phase of the image as a result of the FT.

It's a bit like saying "why do I need to perform an addition before calculating the sum of 2 number?".

I would suggest you to learn more about Fourier Transform