Simply put:
- QGIS is like a desktop Excel (very powerful and capable of doing almost everything, but it works locally on a single computer)
- Usemaps is like Office365 (also very powerful and able to do many things, but designed for multi-user collaboration and used through a web browser)
Usemaps is a System you need when:
- you need a tool for easy collaboration with many people,
- when you see that Your colleagues do not want to work in QGIS,
- when you simply want to add an attachment to an object (in QGIS this is not so straightforward)