They're working on an expansion for Factorio, and this refactor may have something to do with that.
Maybe they just want to leave their code in good shape so they (or someone else) can come back to it at a later time and pick it up relatively quickly.
Expansion packs typically depend on the base game, so improving the base engine is likely directly contributing to the work on the expansion. The expansion is likely changing things about the base game, and they'd want tests to assert both with & without the expansion are still working as intended.
