Why are there open primaries? What possible benefit do they serve?I'll start with the punch line: for the most part, open primaries don't seem to make much of a difference. That's because primary elections are low-turnout events in most cases, and so the most informed, attentive voters are the ones who tend to vote in them, and they tend to be partisans (and only a very tiny number of extremely high-information voters are likely to raid the other party's primary in order to deliberately try to mess them up).
As far as why they exist....the most common reason given is anti-party; some people believe that it's undemocratic for a smaller group of people to select the candidates and to therefore restrict everyone else to a narrow choice in the general election. I disagree with that, as long as the parties are permeable and relatively non-hierarchical (that is, as long as the parties are internally open and democratic).
As a practical matter, some states have no party registration, so they more or less have to have open primaries.
There's actually one reason for open primaries that I would support: if the parties prefer them as a form of advertising, or as a means of affecting outcomes (I can imagine circumstances in which that could be the case).
In principle, however, I think it's very good for democracy for parties to control their own nominations, so if they want closed primaries I support their right to do so.