Excel is a powerful tool. I was solving system of differential equations with Newton method in it. Sometimes it is easier than in Matlab (or Mathematica) if all you have is good understanding of how step-wise equations should look like, but not the differential equations themselves. Those steps may include if statements, for example.
Had to do a similar project and it took me three full days of back and forth with another software before I found out EXCEL rounds small numbers in very weird ways.
Also, in EXCEL functions/formulas and data/values are wildly mixed.
(Not mentioning a plethora of other mildly infuriating quirks here)
Kind of serious, the lack of smooth scrolling makes Calc really horrible to use on a touchpad or with large/differing sized cells (formatted sheets with headers and such)