Why choose 120 volts?

Seems like if the c14 was connected to a 240v PDU the 5-15 would deliver
240v to the equipment, arc/pop tripping the breaker on the PDU as soon
as it is connected killing power to everything on that PDU.

Well, the latter half of that is making all sorts of assumptions. Your
typical 208V 20A circuit is capable of delivering watts roughly equivalent
to a 120V 30A circuit; if whatever is being destroyed is beefy enough to
momentarily short out that much power, yes, the breaker should trip, but
in many cases, a power supply might only be designed to handle a hundred
watts or so, and who knows what component leads, PCB traces, components,
etc., might burn out quickly before the PDU breaker really "notices."

Or am I missing something, Also hard to believe it is UL since the c14
is rated 125/250v and well captain obvious says the 5-15 125v max.

It is UL when used in the intended manner. Note that it is marketed as
a monitor feedthrough power adapter (or something like that) - not a
208V-to-120V converter dongle.

There are many examples of things you *can* do that are hazardous with
UL listed devices. UL listing mainly means that something isn't all
that dangerous (from an insurance perspective, at that).

Consider, for example, that you can plug a household extension cord
(UL listed, 14ga, ~13A @ 120V) into a 20 amp circuit, and then plug
in 16 amps worth of stuff, and watch the cord heat. It isn't UL
rated for such use.

... JG