If there were a few percent more, plant life would burn up. Mars and Venus have virtually no free oxygen, so necessary to most kinds of life. Earth's gravity is just right. If the earth were only one-fourth as massive, the atmospheric pressure would be too small for life. If the earth were twice as massive, its atmosphere would work like a greenhouse in summer, raising the temperature enough to kill us all.
Earth has the right kind of sun.
Both sugar and oxygen are needed by animals, and they can produce neither themselves. The sun cannot vary much in brightness or life will not survive. But the creation of plant life appears to be timed just right to save the day. As the sun got hotter, plants removed carbon dioxide from the atmosphere, replacing it by oxygen at just the right rate to turn down the greenhouse effect and keep temperatures in the range safe for life. This performance by the plants only worked because the earth was at the right distance from the sun.