DAX Difference - Measure as variable vs. Measure directly in formula

[GSE]

Hi all, 

I would like to understand the difference between declaring a measure as a variable vs. using the measure directly in the formula without declaring it as a variable first.

What I noticed is that in the first example, the processing time is almost 8 times higher (when not declaring it as a variable) and in the second example the formula throws an error.

I know that if a measure is declared as a variable first, it is evaluated in the context of the visual but how is it evaluated when not declared as a variable and why does it take so much longer or throws an error.

Example 1

not declared as variable (processing time almost 8 times longer)

declared as variable

Processing time (first matrix is with declared variable and second matrix is without declared variable)

Example 2

not declared as variable: error

declared as variable: no error

[lbendlin]

The fun part here is that "variable"  actually means "constant".  You are basically turning the measure into an immutable value that will now no longer be recomputed in the following computations.

Power BI has no concept of (mutable) variables. These are all constants.