Research conducted on existing digital products to evaluate how they fit into a user’s life to help them accomplish their goals. Evaluative research uncovers a user’s goals, challenges, workarounds, and ideals for using a product in order to understand how the product can be improved.
Here's an analogy.
Evaluative research is like the scientific method: You develop a hypothesis and test it to either confirm or disprove, and depending on the results, you make changes.
Why does it matter?
Validating a product or feature with your users ensures you built “the thing right,” since ideally you are already building “the right thing” (because you did generative research).
How does it apply?
Validating a product or feature will ensure that you’re putting something in the world that users want, can use, and will enjoy.