Docs: add some text on why one should use modelspec (what's the usecase)?

[sanjayankur31]

I think it'll be beneficial for us to add some text to the documentation that addresses "why should I use modelspec"? The examples are there, but they answer "how to use modelspec" more than "why".

The overview for attrs, for example, does a good job at this:

https://www.attrs.org/en/stable/index.html and https://www.attrs.org/en/stable/overview.html