alpha is a cinematic medley of several iconic minecraft tracks (namely, 'minecraft', 'mice on venus', 'moog city', and 'sweden'). (don't get it confused with the OST album. alpha the song appears on the album volume beta but is a medley of volume alpha songs).
the soundtrack for minecraft is intentionally designed such that it is able to fade into the background most of the time, except for when it accentuates focal player moments. (this might be why a lot of people claim songs tend to play more often at sunset or sunrise -- they don't, it's random. but i digress). therefore, the songs in minecraft might come to represent specific memories, unique to every players experience. a particular song might represent, to someone, the memory of building their first house.
so, at the 'end' of the game, while the beautiful end poem (theendpoem.com <- go read it if you haven't) scrolls by, 'alpha' is the song that plays. 'alpha', which is a medley of those other songs, reminds the player of everything they've done to get this far. those same familiar songs, but with even more feeling and elaboration.
this song is so beautiful it's made me cry more than once.
the only thing this song could have against it is its length… but is that really such a drawback?