pre-out voltage

im a noob

i have been looking at head unit specs and i see that the preout voltage goes  from about 2-6 volts, why is there diff and why do i need more?

You will see those differences in output voltage because the head unit manufacturer has just a few guidelines to follow, one being to publish specs that have been tested.  But you often won't find the frequencies that the output voltage was tested at. 

Say you are setting up two amps off that HU, one for the components and one for the subs.  The amps have gain controls to adjust the amp's sensitivity to the HU voltage output.  The HU may be putting out 4 volts at a frequency of 1 kHz, and you adjust the amp gain to that level.  Now, that same HU might also put out 2 volts at 100 Hz, but there again you have another gain adjustment to make on the sub amp for that particular output.

You don't need more (pre-out volts), but the amplifier gain is set lower if you do have more.  Which means the signal is amplified less, so any noise that is inherent in that signal is therefore also amplified less.