Dear Sid Abdelghafour , I moved your question to ChatGPT as an experiment. As a result, I received the following response from ChatGPT:
"Grounding the slot in a microstrip patch antenna improves gain and efficiency by reducing the amount of loss in the antenna. The grounding helps to reduce the surface waves that can cause loss in the antenna, and also helps to increase the directivity of the antenna by reducing the amount of unwanted radiation. Additionally, grounding the slot can also help to improve the impedance matching of the antenna, which can further improve its efficiency."