Think of namespaces as partitions that don't go away when you secure erase the SSD.
The controller and namespace(s) are separate. Namespaces can be shared, and can allow remote controllers to access them. Companies like Excelero or Lightbits leverage this technology with their product offerings I believe.
In large devices, say 15TB NVME, you may want to carve the SSD up into 3 separate pieces for different purposes (Cache on /dev/nvme0n1, Database on /dev/nvme0n2, VM storage on /dev/nvme0n3). You could create 3 equal or unequal sized name spaces and use those on each /dev/nvme0n_ device. Any space left over goes to over provisioned capacity which has its own benefits.