Players can even do some role-playing that would take more time then in a direct Chat style environment or that they prefer the other players to be unaware of their character's actions. The focus of such a usage of Email or BB is to reduce the record keeping of the adventure within the chat room. This will reduce the amount of extra clutter you would spend time on within the chat room. Worse You need to be much more prepared. Everything takes more time to accomplish.
Players can be very unreliable on the Internet.
You have to describe everything in detail which means plenty of typing You are not in direct contact with the players and can not easily engage their emotions. Best You can meet players from around the world. You can write a story and have the time to edit it the way you want. It also provides another good way for people to fit role-playing into increasingly busy schedules or do not have a good local group they belong to. Just do not expect it to be the same as the table top gaming you are used to playing.