if you vote for someone outside of the main two parties your vote automatically doesn't mean anything outside of weakening one of the two main parties by taking votes from them (which admittedly can have an effect if the third party is powerful enough, but this requires people to vote for them in the first place)
otherwise you're just throwing away your vote. at least by voting for one of the two main parties you're voting for a party that actually has a chance to win
my definition of mattering is actually having some sort of affect on the outcome of the election, so voting for joe bogo the cat won't matter at all unless enough people vote for him so to take a large percentage of votes away from the big two parties
i love first-past-the-post systems