Salaam why muslims kiss the black Stone ?
Alaikumassalam wa rahmatullah,
The Prophet did it, which is why people think it is something beneficial to do. Some hadiths mention that it is a stone that comes from Paradise. Maybe kissing it is just a way of trying to feel closer to something that has some sort of relationship with Paradise and God. Umar ibn al-Khattab is mentioned in a hadith as saying the following before kissing the Black Stone:
By God I know that you are only a stone, and if I had not seen the Prophet kiss you then I would not have done it.