I'll also summarize the advantage of equal temperament:
Regardless of what key the song is in, a certain interval is always the same exact ratio. A major third in the key of F is the same as a major third in the key of Bb. This is good for instruments like the guitar and piano, which aren't made or tuned for a single immovable key. Contrast that with harmonicas, for examples, each of which is made for only a certain key.