Some things are just easier to calculate in JS than in Coda. For example, running some while-looped calculations, e.g. calculating task end dates based on start dates and people’s complexly set up availabilities would happen much faster in JS than within an iterator table or a fickle forced-recalc formula
Also it may serve those teams where most of the Coda users are not skilled enough to come up with complicated formulas and algorithms, yet they want to not just use the doc they’re given but extend it with calculations that matter to them. The team could employ a JS coder to prepare some pack functions and only that — faster for the coder who doesn’t have to set up everything on the Coda side, and the not-so-tech-savvy team are happy to just go and use those formulas.