Hi Hamish,
With the current release of Collector for ArcGIS (v19.1.0) we support offline editing and sync with the Default version just as ArcGIS Pro does today. This can be quite useful for field data collection workflows where you are capturing new information directly into your enterprise geodatabase.
Currently planned for ArcGIS Enterprise 10.8.1 is support for offline editing and sync with a branch that you have published. This will then bring forward the maintenance workflows where you want to institute a QA/QC process and perhaps manage potential issues where multiple field workers edit the same features.
Thanks!