Cream

Culinary definition of cream:  To whip or beat butter, shortening or other fat quickly until light and fluffy, with or without the addition of sugar or other ingredients.

C@H FOOD FOR THOUGHT
There isn't much we can add to the traditional explanation, other than DON'T cut time and corners by using melted butter and sugar, instead of creaming room temperature butter and sugar, or yes even working not-so-soft butter into a creamed state by using good old-fashioned elbow grease or a counter-top mixer. 

Your recipe won't turn out the same if you change the physical state of the butter from solid or semi-solid, to liquid.  There is little (no) measurable advantage to creaming by hand instead of with an electric hand or stand mixer - other than you can 'feel the love'.

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