The ability to hear that a song needs more treble or more bass to sound better. The technical training will teach you to recognize what you're hearing. In other words instead of just saying that song needs a little more bass, technical training will allow you to know what frequency you're hearing so that adjustments can be made. The engineering aspect of it deals primarily with the frequency of the sound. That's the most important thing that must be mastered.
Engineers constantly have to adjust how they hear music.
Digital is more precise but there's a lot more to it. It has more editing capabilities. However overcoming what I like to call digital noise is a problem. That in the industry is called golden ears. I'll give you an example.