1) You don't, unless you care about knowing the value
2) You can, if you don't care about outside influences
3) Because you want more than one that's the same value
4) Because bandgap is the best you can fo, at low voltage,
Because voltage is not current, and needs to be transformed
to get the result sought.
All of these are really, really fundamental questions whose
answers could probably have been found read in the time
you were waiting for the grapes o' wisdom to fall into your
mouth. And you may only get a raisin.