Difference between internal-sftp and sftp-server?

I created a restricted user with rbash, for sftp file sharing between desktop(client) and laptop(server) but whenever i entered password it would return "Connection closed" in progress of connection. Other users worked without a issue. After a bit of research i changed Subsystem config of sshd_config to internal-sftp from /usr/libexec/sftp-server. And it was done but left me curious since i have big desire to learn about linux: Why it could not happen with default configuration?