Why is Roy's Identity so important? I've been reviewing some of my microeconomics theory and have been reading up on Roy's Identity. Recall that Roy's identity is defined  as:
$$x^*_{i}(\text{p},m)=-\frac{\frac{\partial v}{\partial p_{i}}}{\frac{\partial v}{\partial m}}$$

being that the indirect utility function is just the utility function evaluated at the appropriate Marshallian demands, this result is not surprising. 

What is an important application of roy's identity that I wouldn't know from just solving for marshallian demands?
When working with assumed functional forms, then you could be justified supposing that there is not much added value.

However, when you try to model behavior without functional forms, you will need all the help you can get in terms of results like Roy's identity to prove your results. 

One important application of Roy's identity is in deriving the Ramsey Rule for optimal commodity taxation. A derivation can be found here on page 14.