It is helpful to have multiples of the breastshield body, the silicone shield&valve and the diaphragm, especially if you are pumping multiple times a day. These parts should be kept clean and sterilised to avoid pathogens.
The high-quality silicone shield&valve plus the silicone diaphragm are crucial in creating good suction. Due to the nature of silicone, these parts will need replacing every few months.
The white cap seals the diaphragm firmly onto the breastshield body - these are essential items and having replacements ready could be handy.
From time to time you may need to replace the tubes and the connectors.