You have 1000 users and a server that can easily do the work 20000 users want it to do each day. Yet, every weekday between 08:00 and 9:00, your server is overloaded.

You buy a second server. Now, your server still is overloaded between 08:00 and 9:00 but fewer people use it between 07:00 and 08:00 and 09:00 and 10:00.

There are plenty of people avoiding the main traffic times for whom the annoyance of congestion is just a tiny bit larger than the annoyance of leaving home an hour earlier. Take away a bit of congestion and they move their schedule.