There are different use cases you can realize by using the VST 3 SDK:
You are a plug-in developerand you want to create audio FX or instrument plug-ins which can be included and used in a VST 3 host application.
an audio FX plug-in is an audio processor effect taking audio as input and creating audio as output: such as Delay, Phaser, Compressor, Reverb, ...
an instrument plug-in is a sound/audio generator, taking as input note events and creating audio as output: such as emulations of well-known hardware synths. There are 2 kinds of instrument plug-ins: virtual sample-based (using audio samples as the basis for sound generation) and virtual synth (using different types of synthesis: physical modeling, additive, subtractive, FM, sample-based, ...)
You are a host developer and you want to load in your application VST 3 plug-ins: audio FX and/or instruments plug-ins.
Advantages of using VST 3 SDK
By using VST 3 SDK directly:
you are sure to be compliant with the VST 3 format.
developing your plug-in based on the VST 3 format allows you to support easily new VST 3 features that improve the integration of these plug-ins inside a DAW. Some 3rd party SDKs use only a common layer between all plug-in formats, limiting in this way the possibility for a better integration, for example exclusive VST 3 features: