I can say why I would use it, I don't know that I can answer why you might. I would this because I love D3. I also don't have time to write a lot of it. D3 can be expensive for basic charts to the point where you'd be foolish to use it. D3 is great for complex data visualizations where the expense is an investment that will pay you back in higher performance and higher flexibility.
For me, I can use plottable and then make the charts my own with raw D3.
For me, I can use plottable and then make the charts my own with raw D3.