The sun is of course very important for the sustaining of life on our planet. If the sun were to suddenly disappear then all life on our planet would certainly cease to exist. During the day the sun gives us energy in the form of light and warmth. The energy from the sun is in a form that plants can utilize via photosynthesis.
Just as important as daytime is nighttime when our part of the earth is exposed to the dark coldness of space. This is when the earth expels the energy that it received during the day. If the earth did not expel this heat at night then it would become a very hot place indeed and disorder would greatly abound.
The earth is not an isolated system, and it is not a system that is only having more and more energy from the sun pumped into it. The earth is a system with energy of a useful form coming in on the sunny side and heat exhaust of a less useful nature leaving on the night side. It is more like a car engine, with useful energy in the form of fuel coming in and less useful energy in the form of heat going out the exhaust pipe.
And so we have our first example of how balance is important in life. If the Sun
shined brightly overhead all the time bathing us in it's wonderful light and warmth we'd
just get hotter and hotter and eventually die from excessive heat. Likewise if the cold
darkness of night reigned constantly supreme we'd just get colder and colder and
eventually die from lack of warmth and light. Life is only good when it is in balance. We
need both the day and the night. We need the constant cycling between them.
(Note you can make out the Big Dipper among the stars in the upper left of the animation at top during the night part of the cycle. The animation here lies by showing all the stars as being stationary at night. Actually the stars move at night just like the sun moves during the day.)