is morale and approval rate important?
what does it give you other than not rebelling?
what are the benefits of high approval rate?
Planetary morale above 75% gives you a 25% population growth bonus. Planetary morale at 100% gives you a 100% population growth bonus. This bonus is multiplicative with all other bonuses, not merely additive.
Population won't grow if planetary morale is below 41%. Population decreases at the rate of 10% a turn if planetary morale is below 21%.
It's difficult to win an election with less than ~55-60% civ-wide approval, with 65% giving you an almost guaranteed win. This doesn't have the effect that it should, however, as your civ only cares what its approval is for the turn of the election, so you can run higher taxes for all but two weeks out of the year.
See also: Population and Approval