In my view, it's the person who is able to understand the business problem the customer is trying to solve, then evalute if the problem is something you or your company can and want to solve. If the problem is something your company want to solve, provide the technical evidences during the due diligence process to the prospects that you can solve their problems. In my case, this also bridged nicely to help out the existing customers to get new things done quickly.
In cloud software at least, sales engineers/solutions architects work with the account manager to drive adoption of services. A lot about evaluating a customer's problem space or existing architecture, and proposing possible cloud-native architectures.
Sales engineers also do educational sessions and service-specific "immersion days" for the platform, which usually tie into the customer account manager's sales and advocacy for the platform, and helps to build relationships with the customer's engineering team.
