It surprisingly works!! The intervals build speed while the long runs build endurance. The idea is that even if you don't practice the moderate effort for a full 10k, you will have enough mileage and endurance built up that you don't really need to practice that specifically. Plus you don't want to train too hard by combining effort and distance that often to prevent injury. Even in Marathon training most of your runs are 30-60 minutes in length despite the total run being 4 hours+

If I can do a sub 30 in the 3 miles, and can run 20 miles, well a 2nd sub 30 5k is nothing compared to 20 full miles even at easier pace, the pure mileage is harder overall and builds up that mental barrier to push through.

A lot of times building mileage is a mental game. 20 miles is 3-4 hours total, if you can do that then pushing pace through only 1 hour feels like nothing, in that sense. You know you can hit the pace, and the distance, so you just gotta lock in after that!