Your plant is only capable of moving so much energy around. If your lower branches are two times farther away from the light than the top branches, they're getting 11% of the energy the top is getting. To an extent your plant can transfer energy downward, but that consumes energy as well. Killing weaker branches near the bottom aids in making your upper branches fatter. I haven't ever ran a test on the subject, but the "grandmaster" who helped me out when I was learning did. He gained, I can't remember exact numbers, it was 5-25% more dry weight on plants with weaker branches chopped? I know it wasn't higher than 25%, but even 15% is a HUGE jump.
Edit* He ran two trials. Zero pruning vs what were talkin about, then partial. Outperformed each time, obviously the higher numbers were from a totally unpruned plant.