Scheduling emails allows the ability to have the email send at a certain time.
For example if I am a retailer who has a promotion starting at 10:00am you want the email to land as close to that time as possible. Scheduling can support this.
If you have the flexibility, it's better to schedule mail for off-peak times. Most emails are scheduled and sent at the top of the hour or half hour. Scheduling email to avoid those times (for example, scheduling at 10:53) can result in lower deferral rates because it won't be going through our servers at the same times as everyone else's mail.