Another reason for the use of a DI is that it takes a two conductor unbalanced line and converts it to a three conductor balanced line. This is a separate issue, not to be confused with impedance. It is a common misconception that all lines that use regular tip/sleeve 1/4” guitar cord type connectors or RCA Phono connectors are high impedance. Not so. The output from a guitar that has a battery operated active preamplifier or pickup system will be low impedance in nature and so will the output of an electronic keyboard, guitar preamplifier, guitar effects processor. The same goes for the RCA phono output from CD players, tape decks, etc. The signals are unbalanced but LOW impedance in nature.