https://ancientgraffiti.org/Graffiti/
Graffiti is a population's expression of ownership of their city. It's a very common form of countercultural resistance and therefore an important relief valve. It's a way for anyone to express themselves on their environment. A city only has value because it's occupied by many people, and those people need to express their autonomy and quite literally "leave their mark."
Not to mention, it's lovely to be connected to a common thread of humanity over literal millenia. Just as I scrawled onto a bathroom stall in 2005 "Cameron takes it up the bum," so too did Salvius write of his friend on a wall in the House of the Citharist in the year 79, "Amplicatus, I know that Icarus is buggering you. Salvius wrote this."