My anecdotal experience (and I'm not remotely close to Marc Andreessen in wealth and power) is that there is a lot of benefits in structure and schedule.

Knowing how big part of your day will play out is a very powerful thing. It removes so much stress and uncertainty.

In the end of the day it's a question of balance. It's probably unhealthy to plan every single hour of your life in advance (which Marc does not do), and the same goes with allowing too much spontaneity.