Roman “graffiti” is famously bawdy, but many of the surviving examples are essentially just advertisements. The eruption that encased Pompeii in volcanic ash essentially froze time for this medium-sized city. Inscriptions such as this one was only meant to be temporary, which gives us an interesting glimpse into how and why the typical Roman actually made letters.
There were actually two Latin languages which evolved side-by-side. As early as the 200s BCE, evidence of a non-traditional writing was emerging in Italy. The highly technical and official Latin spoken by scholars and the upper class was growing into something new in the streets and alleys of
the city, as well as the distant countryside. Classical Latin culture rejected anything Greek due to deep animosity between the people dating back centuries. Linguists and poets rejected any co-mingling with other languages, but the diverse cultural mishmash that made up the lower classes had no such hang-ups. The living language that was Vulgar Latin was free to experiment.
If you consider these three examples, it is clear that the top and bottom examples are rough and unconfined by structure, only by the limitations of the wall. Classic Latin had no lowercase type, obviously. It also has no punctuation and often no spaces between the words. Classic Latin almost seems decorative and needlessly esoteric, where the vulgar graffiti perhaps follows actual speech patterns.
Etchings such as these highlight one of the biggest factors affecting the practice and advancement of graffiti. Writing illicitly is difficult and time-sensitive. The graffito-tagger must work quickly, thus simply. Noble, stoic, evenly spaced high-Latin letters become erratic and random as they meander across the wall. The amateur has no concern for leading, line length, x-height, or any of the conventions we take for granted. This tendency for random roughness will become the stylistic highlight of all graffiti moving forward, however truly memorable works will always have a well arranged balance and evenness despite breaking convention.