15ft Deep Sump Pump Hole
To remove water below the basement.

To provide a significant amount of storage in the event that the pump fails, allowing some time to correct the problem (depends on the inflow rate, but more depth is more storage.)

Depending on the age of the house, it may actually have been the well supplying the house at one time, and was converted to a sump pit when the water supply changed to an alternate well or municipal supply.