There is a discussion playing out in news headlines right now that underscores the reason for a seemingly odd practice in at least some Baptist circles. As you have probably seen, there are deep divisions in the GOP over Donald Trump serving as the presidential nominee of the party. Indeed, opposition to Trump in party ranks is so strong that some delegates to the Republican National Convention are pressing for a change in the rules allowing them to vote their conscience so they won't be forced to vote for Trump.
In general, delegates can't necessarily vote according to their own conscience. By definition delegates attend a convention with delegated authority from some other body to vote according to the instructions of the body that sent them. Delegates are expected to vote according to the instructions of the sending body whether or not those instructions are aligned with their consciences.
In the Baptist circles of which I've been a part for much of my life, those who register to vote at Baptist conventions or general assemblies are known as messengers rather than delegates. The reason for this seemingly odd terminology is linked to the doctrine of the priesthood of all believers. One of the implications of this doctrine is that we believe it is wrong to coerce the conscience of a fellow believer (or anyone else for that matter). Messengers, unlike delegates, are expected to prayerfully vote their own conscience at a convention or general assembly.
Every year it seems there are examples of the press labeling voters at Baptist conventions or general assemblies "delegates" rather than "messengers." Indeed, I've heard quite a few long-time Baptists make this mistake. Certainly there are more pressing issues facing us than the names we use for convention voters. But it isn't often that we get such a good teaching moment to highlight the fact that many Baptists use the term "messenger" rather than "delegate" and the reason for the difference.