As a Sales Engineer, why should you have technical skills like advanced knowledge of the products and problem solving skills?

To convince and satisfy the potential customer about the functionality, usability and maintain ability of the product.

If you are working in a product company as a sales engineer you must have technical knowledge of the product, since you are the first personnel contact of the organization facing the customer. The customer may ask information not only on sales (cost/servicing and other facilities) related, may be they may ask you to interact with their technical team so that they can analyse from your answer whether your product can meet their requirements or not.

Based on your response on the interaction with the technical team, they conclude whether they can customize the product to meet their requirement if the basic product is very general one.

Also when you have a better problem solving skills like the customer might have explained you an issue they faced, and when you have the technical knowledge you might be able to give a suggestion and the technical team may get some idea based on your answers. This will create a good impression about the product and the organizations talented resources.

This way it will help your career growth and also a respect as a technical knowledged person and not a person with only sales figure data.