I saw a poll stealing my thunder about what I was gonna predict so I decided now was the time to do so before the necessary threshold is met. That is, if Bernie wins Iowa and NH he will also win SC. Anyway, with that out of the way let me explain the best case scenario for Iowa for Bernie.
1) Bernie Wins in an overwhelming fashion.
2) Pete Gets second place
3) Warren gets 3rd
4) Biden gets 4th or Below.
Bernie winning in a big way speaks for itself, he gets a lead and he also gives americans a reason to believe he is the most electable because he is.
Pete 2nd over Warren is important as well, Pete has no minority support, regardless of how well he does in NH and Iowa he's not going much further than that. Him beating Warren will lower the amount of people that believe Warren can win and possibly lower their enthusiasm for her causing them to move on. Warren supporters 2nd choice is Bernie Sanders, this is a win. Warren and Biden together is the only thing stopping Bernie, if either 1 drops out Bernie wins the nomination with ease because he is their 2nd choice.
This leads into Biden. His entire argument is that he's the most electable. If he comes 2nd or 3rd, he might be able to keep people thinking he still is, but if he gets 4th or below? He's done. His support will crumble, his "firewall" will break and his campaign will collapse by super tuesday. with Biden out of the way, his supporters 2nd choice will move to first and that is Bernie Sanders. With that done Bernie would have successfully set himself up to sweep the primary and win on the first vote without super delegates getting a chance to steal it from him.
In other words, if we win tonight, and we win NH and we win Nevada, we will win SC and with an early states sweep we'll have our path cleared into super tuesday. The real final boss that will try to prevent us from getting the nomination on the first ballot is Bloomberg. He's only running because the establishment figured Biden would collapse. If He ends Iowa in fourth, that's it. We've possibly won the nomination at that very moment.