When to use qmake eval?

https://doc.qt.io/qt-5/qmake-test-function-reference.html

eval(string)

Evaluates the contents of the string using qmake syntax rules and returns true. Definitions and assignments can be used in the string to modify the values of existing variables or create new definitions.

For example:

eval(TARGET = myapp) {
    message($$TARGET)
}

I wondering when/why that should be used? Can't we just use TARGET = myapp directly?

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