Lawfuls get an alignment penalty for writing graffiti with a marker, even if it’s Elbereth or something else useful.

Convicts get an alignment bonus by writing graffiti. (Graffiti has to come from a magic marker, or a felt marker in variants that have it.)

Some random graffiti is not rumors; it contains helpful in-game information. Things like “NR 9 is a scroll of earth” (giving object identifications), or “The Big Room is on level 12”.

Graffitist role, in which your job is to go around making graffiti everywhere.

You can engrave on walls or doors. Elbereth on walls or doors doesn’t work. You can read it by standing on the nearest space. Random graffiti, if it generates on a floor square next to a wall or door, may say “Something is written on the wall here.” instead of the floor. Graffiti could render as ~ or something similar.