It's often useful to display a measure of spread when presenting an average.
the average revenue per customer per transaction is \$100, meaning that (assuming a distribution roughly centered around its mean) the typical customer spends around $100 for each transaction he makes
the standard deviation of revenue per customer per transaction is \$10
This way, we have a pretty clear understanding of the overall distribution, i.e. we know that the revenue per customer per transaction typically fluctuates in a range of around 10% of the average revenue per transaction.
But now, consider this other example:
the weighted average of revenue by customer is \$150, when using the count of transactions for each customer as a weighting parameter (e.g. a customer with 100 transactions would count for twice as much as a customer with only 50 transactions)
what would then be a good measure to give a notion of the spread?