The value of an expression will be plotted separately for each dimension value. For example, if the dimension is "Year" and the expression is "Sum(Sales)" each Year point will have a different value.
A reference line is plotted at the same value across all dimensions, and can be used to show something like a target value that would be the same for all Years.
I suspect you know the above and asking about using a constant expression as a reference line. The differences I can think of are:
1. Reference line has a label property.
2. Reference line is not "data", therefore would not get exported to excel or scale the chart.
