Gratuitous

Etymology

From Latin "grātuītus," "given freely, without payment," from "grātia" ("favor, grace"). The original sense — freely given, costing nothing — is still alive in "gratuitous advice." Because something given for no charge is also given for no necessary reason, the word acquired its critical sense of "unwarranted, needless."

adjective · /gruh-TOO-ih-tus/
Uncalled-for and unjustified — done or included without good reason.
What it names
Use it for what serves no purpose and could have been left out: gratuitous violence in a film, a gratuitous insult, where the objection is that nothing warranted it.
Example
The scene added nothing to the plot; it was gratuitous cruelty for its own sake.
Related idea
Compare superfluous: the superfluous thing is simply more than needed, while the gratuitous thing is unneeded and faintly objectionable for being there at all.