Sorry Buzz Box and others but in this case I think EMIT is right on the money, to most professionals 'Oceanic Track' refers to the Atlantic tracks. Pacific uses airways, not tracks, as does the Indian Ocean. There are specific rules for flying the Oceanic tracks. ATC assigned Mach no. is but one.
You burn more fuel so you can climb higher to save fuel.
At the beginning of a thirteen to fifteen hour leg, for example, if you think you are going to get trapped in the lower levels, I can see that working.