A few things that I use it for: Pre-programed D&D stat trackers because it's free and Pathfinder is... complicated. Factorio base ratio calculations, and EvE online related market data. Quickly sharing database query output. A simple weight tracker.
Most of these things are and can be independent services, the issue is if I need something now that's ad hoc, then Sheets provides a computation sandbox in a format that most people are familiar with.