I've used it for an insane amount of things, starting with home automation and telemetry but also fairly sophisticated uses, including:
- Sniffing for multicast streams on a LAN and exposing IPTV set top boxes as HomeKit devices
- Automating image conversion and video transcoding
- Building an RSS aggregation and transformation pipeline that does translations, scraping, etc.
- Building a simple GUI for managing backup snapshots and batch file copies
- Monitoring Azure deployments (and exposing the data as simple dashboards).
- Doing App store price checks and sending Pushover notifications when an interesting app goes on sale (backed by SQLite).
- An embarrassing number of Slack and Telegram bots for various user groups/events/etc.
As a general rule of thumb, Node-RED can do pretty much any kind of automation you can get a REST API for, and quite often someone out there has already built a module for it.
I think of n8n more along the lines of a graphical workflow/BPM tool for SaaS, but with much less flexibility/ability to go "low level".