Unbalanced Designs - Any Use?

Hi,

I'm currently studying Design of Experiments and I just came across the
concept of Balanced and Unbalanced Designs. I can think of a lot of
reasons/situations when one may use a Balanced Design but none for an
Unbalanced Design!
Do Unbalanced designs have any importance in practical problems? Are
there reasons/situations where an unbalanced design may be more
advantageous to use than a balanced design?

Thanks!
Abhishek

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