The Spoiler Effect quickly causes a first-past-the-post voting system to devolve into a split two-party rule. This leads to an increasing number of people being unhappy with the less and less palatable candidates presented by the two ruling parties.
Under the alternative vote method, the voter selects candidates from most favorite to least favorite. Then votes are counted, candidates eliminated, and votes redistributed, until a single candidate has a majority. This leads to more, smaller political parties, and candidates that actually have to work to earn your vote. More, higher quality options for office.
This is a simplified explanation, and more information can be found in the Politics in the Animal Kingdom series on YouTube. We need this now more than ever, as a nation.
Thank you.