An Application of Consumers' Surplus Here is an example of an application of consumers' surplus. Suppose D is the demand for VCR's and p is the price for which they sell. Before VCR's were introduced, consumers of course got no benefit from them at all. After they are introduced, consumers get a surplus indicated by the area of the shaded triangle. That is their net benefit from buying VCR's and is the consumers' benefit from the introduction of the new good.
People demand different goods and services for all sorts of different reasons.
The economist's strategy is to abstract from all that. That's just what the utility approach does, and it shows that we can say important things without worrying about all the reasons why people buy things. It gives a reason why the demand curve is downward sloping, namely, the law of diminishing marginal utility. I have some problems with this whole utility approach. In your examples, the utility of cokes depends only on the consumption of cokes.