Here is a theoretical problem. It could apply to many systems. Let's assume RESTful Web services for example. You want to find out something but as it happens you can't get it in a single query. You can make multiple queries. For example, you have a joint bank account (or a company bank account) and you want to find out how much money is in there. Meanwhile someone else transfers money from checking to savings.
Checking: $5,000 Savings: $3,000
Me: Query Checking. Response: $5,000
Joe: Transfer $4,000 from Checking to Savings
Checking: $1,000 Savings: $7,000
Me: Query Savings. Response: $7,000
How do you avoid anomalies like this? The example shows two accounts at one bank, but it can also happen with two accounts at two different banks.