How to create precision reference 2.5 voltage
A precision voltage reference is everything that a 7805 and resistive divider isn't.

Precision isn't all about how accurate the voltage is, but how accurate the voltage remains over time.

An precision voltage reference:


Has a precise voltage - the 7805 isn't very precise at all.
Is low noise - By comparison the 7805 has massive amounts of ripple.
Has a low temperature coefficient - 10% resistors drift massively in resistance at different temperatures.


So if you want precision then a 7805 and some resistors are really not the way to go.