The goal of the cutter is to finish with as much weight as possible, while still producing a beautiful and desirable diamond. In the image above, we refer to 40% as the final result, which means retaining 40% of the rough crystal. The yield is often less, due to flaws in the crystal, etc.
For the purpose of simplicity, lets assume each diamond is round, flawless, colorless, and from a well shaped crystal (a topic for future discussion).
Now, since carat weight is the only value factor, our cutter must maximize the final yield to reach his goal. However, if that approach is taken, the finished diamond will be nowhere near the optimum proportions for fire and brilliance.
If cut to the other extreme, each crystal is fashioned to ideal (optically perfect) proportions. The result here seems fine, with one very important exception, value. Diamond (per carat) prices increase at each size increment, half carat, one carat, etc.
So, closing with food for thought: we have two identical diamond crystals with an equally identical cost factor. The first is cut to exact specifications of ideal, and weighs 1.43 carats. The other is cut to the maximum yield possible, and weighs 1.57 carats. In a side by side comparison, the 1.43ct. ideal cut would be a FAR prettier diamond. Anyone’s eyes would be attracted to it, hands down. To the casual shopper, the 1.57ct. will sound like a much better value, and sellers of substandard cuts never show them side by side.