Most of the time you can easily divorce the values needed in an expected form and an actual form of an expectation. In those cases, nothing needs to be shared and your test can use a simple bare expect. However, there are times when you need the same value in both the expected and actual forms - and a bare expect doesn't easily provide with a way to accomplish that.
In version 1.4.16 or higher of expectations, you can now use the expect-let macro to let one or more values and reference them in both the expected and actual forms.
Below is a simple example that makes use of expect-let to compare two maps that both have a DateTime.
If possible you should prefer expect, but expect-let gives you another option for the rare cases where you absolutely need to share a value.