Because I'm an old man with a poor memory & I'd like a gauge to tell my dumbass to stop at the pumps, when the aforementioned Sensei is not with me, before I run out & don't make it to the trail?
On a technical note concerning sending units;
They do work off of whatever voltage system they are designed for (there are some 6v systems) and they are not real conducive to interchange.
They are however designed & operate on a resistor system & develope/operate at different ohms. I'm not familiar with who uses what, but I do know that Ford & chevy are different. Also all the HDs I've worked on, some for certain years do not work with other years. It's hunt & peck, or test with ohm meter & specs chart to make fit.
As the float rises & falls it developes & transmits a different voltage to the gauge causing it to rise or fall in accordance with where the float is. If the 2 are not conducive to each other, you will get incorrect readings.
Contrary to what some of you TBers may think, I know more about all this shit than you think I do.