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Mid-late game it becomes more useful - your economy is stronger and should be able to support 5 units being produced at once. Earlier in the game it's true that you may need to balance your resources more, and build units individually based on what's available. Also, when being attacked, it's safer to build a group of units than an individual unit that may be killed instantly by ranged weapons. In a larger group at least some of the units may survive to retaliate and take out the attackers.