Think about using a photo camera. If you simply take the photo sensor without any lens in front of it you are not getting an image on your sensor. You need to put the objective in front of the sensor to form an image. The electro-magnetic wave in the lens plane and the sensor plane are directly connected to each other by a Fourier transform. A thin lens projects the Fourier transform of the wavefield into its focus plane.
With radio waves it will be quite hard for you to build a working lens in front of your antenna array. But you have a distinct advantage over visible light, i.e. the waves are of long wavelength and slow frequency - therefore you can detect the phase of the waves, a thing that is impossible with visible light. In the consequence you can measure the full information about the radio waves and perform the function of the objective lens in your computer to obtain the image.
So in imaging you always need to perform a Fourier transform, depending on the problem it is either done optically (with a lens) or digitally on a computer.