The silicone cover shields the print cores from cold airflow from the fans, which helps the print cores to maintain a stable temperature while printing. The cover also helps to prevent backflow of material into the print head when something goes wrong during printing.
The silicone cover may wear over time due to the heat of the nozzles. It is recommended to check the quality of the cover at least once every three months.
Examine the bottom of the print head to see if the holes where the nozzles come through are still round and the cover still forms a good seal. Also gently open the fan bracket to check the other side of the silicone nozzle cover.
According to these images, it is highly recommended to replace the cover on the left, and advisable to replace the cover in the middle. The cover on the right is in perfect condition and will not need to be replaced.