Character Analysis
A lot of people in Smith's time wanted to know why there had to be middlemen between farmers and their customers. (Hey, it was a simpler time.) Why should farmers pay money to corn sellers and store owners when they could just sell directly to customers and keep the profits for themselves?
But for Smith, this would make no sense, because farmers would end up spending so much of their time being shopkeepers that it would take time away from them growing delicious food. That's exactly why Smith believes that:
[After] the trade of the farmer, no trade contribut[es] so much to the growing of corn as that of the corn merchant. (4.5.37)
The corn merchant basically gives the farmer the time he/she needs to grow more yummy corn.
On top of giving farmers time to do their job, corn merchants in Smith's mind help make the corn market more stable. That's because these dudes are smart; they'll act conservatively when they think the market will be bad and get bolder when they think the market will be good.
So they end up offsetting any market shocks that might make prices get out of control for consumers. They act like shock absorbers. Or, as Smith writes:
The interest of the corn merchant makes him study to [avoid the ups and downs of the market] as exactly as he can; and as no other person can have either the same interest, or the same knowledge, or the same abilities to do it so exactly as he. (4.5.41)
In Smith's mind, it is always best to have the market being controlled by people who know how it works and how to make it work better.