Say you wanted to write a live audio chat app to talk to your friends. If you'd chosen the Speex codec, then voice would come over fine, but trying to show off your new guitar skills would fail because what they heard would sound horrible.
If you'd chosen the Vorbis codec, then everything would sound great, but there'd be a weird "lag" between participants, as if one of them were on the moon. Also, adding too many listeners would severely strain your internet connection's capacity.
Opus has latencies as good as Speex, and can be high-, medium-, or low-quality depending on bandwidth needs.
Opus is good and not encumbered by patents, and it has just been standardized.