What is the purpose of a dishwasher elbow?
Typically there is very little clearance behind the dishwasher and the back of the cabinet.

A hose attached directly to the inlet might have to be bent and kinked in order to fit.  That would not be good.  The elbow allows the supply line to run parallel to the back of the dishwasher without taking up much room.

As for the ¾" versus ⅜" connectors, again typically the nearby sink would have a shutoff valve with a ⅜" connector, so a standard ⅜" connecting hose would be the obvious thing to use.

It doesn't mean that one can't use other types of hoses, only that one would have to go out of their way to need to.