The presidential primary is where registered voters cast their vote to select the party’s nominee for the presidential election. In a primary, the candidates for a nominated office compete against each other for the votes of the party’s members, and the candidate who secures the most votes becomes the party’s official nominee for the general election.
On the other hand, a party convention is a meeting of party delegates who gather to finalize the party’s platform, decide on party rules, and officially nominate their candidate for the election. In the case of the Michigan GOP, the convention is also used to nominate candidates for other state and local offices.
Therefore, the Michigan GOP uses a presidential primary to allow its members to select the party’s presidential nominee in a democratic manner, then follows it up with a convention to officially nominate the candidate and make other important party decisions.