It is essentially just a management appliance but one that is strongly tied to the VxRail cluster it manages.
VxRail Manager is used for cluster-wide initial configuration, management (e.g. shutting down cluster) and change-management (e.g. upgrading driver+firmware+ESXi).
While it is possible to make changes to a vSAN cluster outside of VxRail Manager (e.g. add storage or nodes) this is not advised (and Dell-EMC state unsupported) as VxRail Manager will not be aware of the changes.
I understand the concept and I believed that answer.
However, since the main layer is VMware (vSAN) and we have the possibility to make changes it is difficult to explain to the end user what he can and can not do in VMware (vSAN).
I understand that vxRail manages the health of the cluster, but it has no visibility to what we do in other configurations, such as VMs and so on. They should basically use cluster-state responses.
And it's also used to orchestrate the creation of the cluster to do node maintenance, as you yourself said.
The question is how far can we make changes to vSAN, without creating problems with vxRail Manager.
For example, could enabling DRS, being an option directly to the cluster, be a big problem?
Only changes such as adding/removing nodes/hardware components etc. may have an impact with regard to VxRail Manager - changes that are normal vSphere operations such as enabling/disabling/editing HA/DRS shouldn't have any impact. Think of it as having an inventory-management role and it makes more sense.