While the website is very handy and provides many useful features, it is self limiting. Providing a public API would allow developers to incorporate your service into their product and allow them to write plugins for their outputs as well as their tools.

For instance, say I'm a software developer who is writing a document and needs assurance that the text will be effective in what it tries to communicate. If this tool where available I would write a plugin for whatever text editor is my preferred one to allow me to use Grammarly to parse the text. Engineers need proper grammar as well.