A convention system for nominating candidates would make it easier for parties to get their policies enacted because it would give them more power over the people who ran for office on their party ticket.
In our current system of primary elections, the party is fairly irrelevant. Anyone can run as a Republican, for example, simply by appealing to the people who vote in the Republican primaries. A person who gets elected this way owes nothing to the party and does not need to obey the party leaders.
If a convention system were in place, party activists would determine who got to run for office. In this system, a politician who didn't follow the party line would simply not be allowed to run in the next election. This sort of control would allow the parties to force legislators to vote the "right" way. This would make it easier for a party to get its agenda enacted into law.