Lot’s of guys know they need a bag to carry their stuff but can get overwhelmed by the choices. One of the biggest questions is: "What is the difference between a briefcase and a messenger bag?" The answer to this questions can vary person to person. But here are the broad strokes.
Historically, a briefcase was carried by a handle. It was a bag lawyers often used to carry legal briefs, hence “briefcase.” It needed to be big enough to hold legal sized documents and secure them. A long time ago, it was common to see hardware/locks on a briefcase. Some still do it. Of course, it’s a little unrealistic because it’s pretty easy to break into one if you want to.
The general idea of a briefcase was a more formal bag used in legal, banking, sales, business and similar settings. It would typically be a more structured bag in shape and not saggy.
On the other hand, a messenger bag was a bit more informal and designed to be carried over the shoulder or cross body. Typically, it was carried by messengers, couriers, etc. It could be a softer bag, usually not as big as a briefcase and often lacked a handle. It would also be quick to get in and out of - a bit less secure.
Over the years, the lines have been blurred. Changes in technology and consumer demands have made it more difficult to tell the difference. However, there are still some rules that seem to hold true. For example, if you’re going to be wearing a suit, you should probably have a briefcase. Carrying a messenger bag with a suit is sort of like wearing sneakers with a suit.
The bottom line is you have to pick what you like and want to carry. Of course, the bag needs to meet your functionality needs as well.