Changing to a convention system from primary elections would allow state parties to enact its positions more easily because it would give the parties more control over their candidates and officeholders.
As things are today, candidates are nominated through primary elections. This means that voters vote directly for the candidates. The state political parties have no real say in who is nominated for any particular position. This means that the parties have very little control over those people. The officeholders (once they are elected) know that they will be reelected if they please the people. It does not matter if they please the political party.
If there were a convention that selected candidates, the parties would have the chance to control the candidates. The candidates would be selected by the political party insiders who went to the conventions. This would mean that any candidate who made the party angry might not get selected as the party’s candidate at the next convention. This means that the officeholders would have to do what the party wanted. If this were the case, it would be easier for the parties to get their agendas enacted into law.