Best way I can explain it is that say you download a large (level 9) grid square at image detail level 17 or 18, you have a lot of high resolution image data.
Each TTC file that GeoConvert generates is a fixed 2048 pixels square.
If you just generated one level 9 ttc file, you're squashing thousands of pixels down into 2048px, i.e. you're losing a lot of detail.
If you generate level 15 ttc files, you'll have lots of 2048px images, i.e. you're keeping a lot of detail.
Obviously the question is then "why not just generate level 15 tiles and forget the rest".
If you're flying higher, you don't need the level of detail you'll get from level 15 tiles, it's wasting gpu, cpu and disk IO to try to render all that. A lower level of detail will give the same experience and be more efficient, maybe level 12 tiles.
In AeroScenery the "Choose For Me" button implements what IPACS recommends in their wiki, factoring in the tile size and image detail level.