From the RePEc blog (Easing the life of referees):

One of the more boring tasks a referee faces is to check the algebra
of a submission that looks promising. This is often trivial to do but
may be quite time consuming, and it requires concentration. Given
programs like Mathematica, the task can be simplified by asking authors
to give their calculations in machine readable code. This would render
it easier to do such boring checks and would, at the same time, help
authors to avoid mistakes.

Mathematica is, however, too expensive. Is there any freeware that
could be used for such purposes? If so, RePEc could recommend using it
as a standard. This would ease the refereeing process. Referees could
concentrate on contents rather than being immersed in ultimately
trivial calculations. Such a program must of course be able to do
symbolic calculations, like forming derivatives or evaluating symbolic
equation systems.

Why are referees of theoretical papers expected to check the algebra? Referees of empirical papers are not expected to check the statistics. Otherwise, we'd be given the data.