Initially, the products or services a company sold were easily understood by sales reps, who could represent these without assistance on sales calls. Any questions a prospect (potential customer) had could be addressed by the sales rep. As technology became more complex, generalists were replaced by specialists.
Sales reps, however, did not develop specializations. This break from a single sales rep being able to handle all aspects of the sale to one which he/she needed assistance was what brought about Relationship Selling, or Strategic Selling. The enterprise sales rep developed skills that got meetings with strategic buyers, but the sales rep was only expected to explain the solution at a high level, leaving the more technical aspects to be explained by the Sales Engineer.
A key differentiator between Sales Engineers and all the other roles in an organization is that Sales Engineers are not on salary and bonus as most people in an organization are. Sales Engineers instead make a base, and a commission.