Use of clamp diode in ULN2003A
It's in the datasheet - https://www.ti.com/lit/ds/symlink/uln2003a.pdf?ts=1596498695539&amp;ref_url=https%253A%252F%252Fwww.google.com%252F
8.4.1 Inductive Load Drive
When the COM pin is tied to the coil supply voltage, ULN2003A device is able to drive inductive loads and
suppress the kick-back voltage through the internal free-wheeling diodes.
8.4.2 Resistive Load Drive
When driving a resistive load, a pullup resistor is needed in order for ULN2003A device to sink current and for
there to be a logic high level. The COM pin can be left floating for these applications.
Your LEDs are not 'inductive', so you don't need to use that pin.
Motors are one possibility.   Driving relay coils is probably the more likely scenario they were considering.