One simple, superficial answer is that Tarantino is a genius at juxtaposing violence and comedy. Just when the violence gets too much, he flips to something absurd.
However there is more to it than that, and the premise of your question seems to me to be entirely incorrect. Each of the scenes you mention does not seem to relate to the plot when you watch it, but they set up another scene later in the movie.
In Pulp Fiction:
The Royale with cheese is perhaps the best example of "non plot related dialogue" you mention. It is a very inconsequential subject. But Jools does randomly bring it up again,
in the middle of intimidating someone, which helps to show what a nonchalant, cold blooded character he is.
The foot massage is clearly related to the plot. Vince argues that his boss Marcellus was justifiably angered that someone gave his wife a foot massage,
but is later attracted to Marcellus's wife, and ends up nearly killing her due to misadventure with drugs.
In Reservoir dogs:
The other robbers are angered that Mr Pink refuses to do the honourable thing and tip the waitress, but he does not back down because he will not be swayed by societal norms. This is in keeping with his character later,
Because he forms no allegiances except to himself, and thus avoids the shootout and ends up with the loot.
And finally another example I'd like to throw in, in Inglourious Basterds (not dialogue but the same principle applies):
Aldo carves a swastika on a Nazi soldier's forehead, apparently of little significance to the plot
but later when he prepares to punish another, more major character, we know exactly what he is going to do.
Tarantino is not the only writer/director to do this (most of them do!) but he is good at it.