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Hi,
D1 is often called a "catch" diode because it catches the negative voltage that arises when the internal switch turns 'off' and the voltage of inductor end connected to the diode goes negative. If it were not for that diode, the inductor voltage would go very negative and blow something out.
The reason for it is also related to the power getting to the output. While the diode conducts, it provides the output with power. That power is part of the total power that gets to the output. The energy is stored in the inductor when the switch is closed, then it is released when the switch goes open. The diode allows that release but only when the internal switch (transistor) is off.
