Descent speed
Longer spent at altitude=Less fuel spent for jet engines
Longer spent at altitude=Greater descent angle=greater descent speed
This increased descent speed equates to less time spent in the descent
Most economical descent speed equals one where the ATC speed restrictions (commonly 250kts below fl100) can be met with thrust levers closed from TOD to touch down. Roughly slightly higher than Vmd (around 280 kts)