User Story Mapping is an important agile technique to help refine the details involved in build something. Be it a whole produce or a complex feature User Story Mapping helps to reveal the details involved, capture sequence of tasks and also estimates of effort involved.
Beyond that though a User Story Map can help to provide focus what matters most as well as who it matters to and how. With some real effort put into a user story map you are more likely to arrive at something your customer will value more quickly. This happens in part because you know who this is for, how they are expected to use it and as such able to relegate some of the nice to have features to future revisions.
With this type of focus you can avoid effort applied to features that do not provide as much initial value to the customer but may at some point down the road. Also, the user story map shows the sequence of tasks needed to be completed to avoid delays on dependencies or late breaking changes.
Once you have a user story map you can import it into your ALM and get the team working on it. Revisit the map as needed to work through the next release of priority features.
To get started with User Story Mapping it is ideal to have some User Personas defined as well as some Design Thinking explorations on how those personas are expected to use you new feature to do their work or solve problems.