Fields are used to report and annotate File, Model, Cell, Element Properties. It would be good to be able to support the use of expressions so the information extracted can be manipulated or used to calculate / derive other values.
Possible uses:

We often work in different coordinate systems that need to be transposed or re-computed depending on the context. An elevation may be extracted and the the z-axis transposed onto the y-axis in 2d file.
We may need to work in an Ordinance Survey / Cadaster datum, but need to report levels/positions in a local project coordinate system.
We have need to label things by using a test or a range. Label item by z-value, if 0.00 < X < 3.5m then 'Ground Floor'.. if 3.5m < X < 7m then 'First Floor'. Or, length / 75mm = X 'brick courses'.
We could use some simple math processing to calculate gradients in section.