Importance Of The Grave
The reason for visting grave has both emothinal perspective and religious perspective. Emotionally, we humans need something tangible to relate and express our relationship with the other person, whether dead or alive, and grave provides for that tangible space where one can feel that emotional bonding with the other person. Secondly, from a religious perspective, it is a constant reminder of the ultimate truth that we all humans have to face one day in the form of death. For the rest of your question, please refer to the thread that I shared above where Ghamidi Sahab has answered all the related questions.