SDB, or Simple Database, was created to serve certain needs that the grains and pillar systems cannot provide. Specifically, grains and pillars are constructs that exist entirely for the benefit of minions, and as such are not available to master-side operations.
This makes SDB more useful for lookups that happen inside the master configuration and for Salt Cloud configuration. SDB data is still available via minion configurations, and there are both execution and pillar modules for SDB, but in those cases, you may find it more appropriate to store your data inside a pillar.
Before we get into all that, let's talk about the use cases that drove the initial development of SDB.
SDB was originally intended as a mechanism for keeping passwords out of Salt Cloud provider and profile files. As you can imagine, keeping the following code block around in plaintext would constitute security risks:
azure-centos72: provider: larry-azurearm image: OpenLogic...