B/c most ICs and sources are designed for an output impedance of 50 Ohms, so we want to ensure impedance matching, so that there are no STANDING WAVES. Try to google out some article on "TRANSMISSION LINES".
That is right. Most of ICs are designed for 50 Ohms impedance. But why these are designed for 50 Ohms? Is this due to IC's limitation? Or is this due to some historical reason?