A convention system for nominating candidates would help state parties to enact their agendas into law because it would give them more power over the candidates. If it were clear that a member of the legislature could only run for reelection if the party liked them, the member would have to do what the party wanted. This would make it much easier for the parties to get the votes they needed to enact their agendas. This, of course, only applies to a party with a majority in the legislature.
In our current system of primary elections, parties have no control over who is nominated to run on their behalf. Candidates need to appeal to voters and to interest groups, not to the party leaders. This means that candidates get elected without owing the party anything. It also means that candidates who have been elected do not need to rely on the party for reelection.
In a convention system, it would be party insiders who selected candidates. This would mean that the candidates would have to appeal to those party insiders, not to anyone else. They would have to promise to do whatever the party said. Once in office, they would know that they could be replaced at the next election if they angered the party. Therefore, they would vote however the party told them to vote. This would make it easier for parties to implement their agendas.