What is the use of crystallized Photosystems from cyanobacteria?
The main purpose of crystallizing a molecule or molecular complex like a  Photosystem is usually to provide a crystal for X-ray crystallography.  A crystal has its molecular components arranged in a structured systematic repeating pattern, and this repeated pattern allows the X-rays to reveal the 3-dimensional shape of one of the components.  Knowing the shape is of biological interest as it suggests how the molecules perform their function.