<p>I'm thinking of another extruder on my printer, and I'm curious about this one....</p>

<p>Is it necessary to have both hot ends on same height? Why yes / why not? (if there is not)</p>

I don't have dual extruder printer myself, but to my understanding having both nozzles leveled at the same height is critical for getting successful prints.
For typical FDM printers, the lowest point of the end effector should always be the nozzle. If you, for instance, mount a fan lower than the tip of your nozzle, it will eventually collide with the printed object.
The effect of having unequally leveled nozzle tips for a dual extruder printer will be exactly the same: one of the nozzles will either drag against or collide with the model during print; or, one of the nozzles will be to far away from the model, giving poor layer adhesion. Either way, the result will be sub-optimal.
So, leveling both nozzles equally is probably a good idea. You might want to have a look at
this question
on some advice regarding how to do it.