OK, the multiple flights to/from hub cities at the same time (or a few minutes apart) are called "wingtip" flights. The theory is that the traditional hub structure has "banks" or "waves" of flights. Some markets have more than one aircraft's worth of demand. So, they schedule two or even three flights at the same time to meet the demand into/out of the hub city. In the 80's and 90's, there was a Sr. Level executive who was a proponent of wingtip services and worked at UA, NW, and US.

Note that "wingtip" service is different from hourly/half-hourly service. "Wingtip" is scheduled to meet the banks of flights at the hub airport. Hourly service is simply to provide lots of flights between two high demand cities.