They typically allow the use of PrefixCommands.
And this has to do with the way communications are handled. On systems like Unix, "dumb" terminals communicate over serial lines, sending and receiving 8 bit ASCII characters a character at a time at a rate of speed determined by the terminal and the connection to the host.
IBM "big iron" uses 3270 terminals, which are block mode: an entire screenful of data is sent when the send key is pressed. In addition, big iron preserves concept of the 80 column punch card. Each line on a screen is a logical 80 column card, and jobs are submitted as virtual card decks, as though physical punch cards were being fed to a card reader.