This is only the case in Texas, which is a primary-caucus hybrid. Other states have either a primary or a caucus. I don’t know what the reasoning is for the hybrid in Texas. I do know that it’s much cheaper to hold a caucus than a primary, so states who don’t have much funds for a primary election will hold a caucus instead. A pro for having a primary is that more people vote in primaries (they last all day and it generally doesn’t take much time to vote) so it’s more representative of the masses; a pro for having a caucus is that, because they don’t last all day and you have to get there at a certain time, and it’s kind of a ruckus, you get really dedicated voters out. While it may sound backwards to say that caucuses are good for that reason, the benefit is that you can measure the amount of activist enthusiasm for a candidate and the effectiveness of his/her ground organization for Get Out the Vote. You want to know who’s got good organizers and dedicated supporters, in addition to winning primary voters, because that will be important in a general election.
What’s cool about Texas is we get to see both, in the same place.