IRV allows the voter to specify first, second, even third, fourth and fifth choices. It means you can vote for the candidate your really like the best, and also for fallback candidates.
IRV gives candidates who may not seem to be so strong in the polls the chance to take on high visibility challengers.
If your first choice doesn't make the cut in the first round, then he or she is dropped, and your second choice is counted. This process of elimination is used until the race is down to two candidates. It makes sure that you get to vote for your favorite candidate and for the final match-off between two or three candidates.
The alternative is to let the media and a handful of powerful politicos play much too big a roll in influencing the primaries.
This is a process that allows voters to vote for their true favorite. It SHOULD be used for ALL elections, and should be combined with laws that give third party candidates equal access to the ballot, rather than the unfair laws that, in many states, force third party candidates to get ten, twenty or twenty five times more names on petitions than Democrats and Republicans.
Getting IRV for general elections will be a very tough process. It should be easier for Democrats to make it happen. The members of the Democratic party should demand it of the leaders.
We need to start doing it now. The technology is simple, but setting in place could take time.