Its hard to say anything about the phase 90 which hasn't been said many times before, this is one of the all time classic pedals and after the crybaby and maybe the tubescreamer this is probably the most famous pedal out there.
On the ground its a very simple idea, duplicate the signal and move one signal in and out of phase providing movement within the tone. Guitarists have been adding vibrato to their playing for a long time for the same reason, phasing is another way of doing this in a different manner. However when players started adding the phase 90 to their songs it produced tones which had never been heard before and as a result sounded out of this world.
Remember, this is the phaser that all other phasers (and many other modulations) are judge by!