The purpose of writing single letter variables IMO is to allow for quick assessments of structure as opposed to meaning. For example it makes nesting more obvio

The purpose of writing single letter variables IMO is to allow for quick assessments of structure as opposed to meaning. For example it makes nesting more obvious. In math there’s also some incentive to elide the meaning of a variable completely, as it often adds unnecessary tethers that hold a statement down from being abstracted. For example if we phrase the axioms of Euclidean geometry in terms of some unspecified object L, we can more readily work with a system that rejects the parallel postulate (e.g. two Ls that aren’t parallel intersect more than once is easier to imagine than two lines that aren’t parallel intersect more than once).