Of course if we asked the oil companies why they don't bother to line up their gasoline molecules nice and neat before selling them to us, their answer would surely be, "Because **** you, that's why!" And then they'd light a cigar with a billion dollar bill.
What will it really do?
Nothing really bad, thank God. Unfortunately it also does nothing really good either. Likely it will just sit there on your gas hose, taking up space, a telltale sign of gullibility for every mechanic who looks under the hood. One glance and they'll know they can charge you $50 to realign the air in your tires.
Assuming for a moment that a magnet can even affect gas molecules, due to the fact that gas is a liquid, it will pretty much go back to being a jumbled mess after passing by the magnet anyway. This is maybe why the Federal Trade Commission actually used the word "bogus" when they gave their report on these kinds of magnets. You know your product totally sucks when scientists and government officials use "bogus" to describe your claims, since that's as close as they'll come to using "bullshit" in an official capacity.