The day/night cycle is important to all races in a number of ways. Depending on the species you are, a few things will be different, but most things will be the same.
For example, at night, many kinds of creeps will be sleeping. While some, like Golems, will not sleep, most will not awake unless attacked by your units. This gives you a first hit advantage which can also be used to immobilize them.
It also effects your line of sight, making it better for humans and orcs to attack during the day, when they can see better, and for the Undead and Night elves to attack during the night, so they can see better.
Also, keep in mind that choosing who to attack is the same. Attacking with night elves on a human base in the middle of the night is to your advantage, since you can see more of what is going on while they can see less.
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