The amounts of water than can accumulate during rainfall.

During a very modest rain event of 1/4", over a 1200 square foot ranch style home, a whopping 180 gallons of water is collected and deposited along your foundation by your gutter and downspout system. 

Expanding on this, a 1/2" rainfall would deposit over 370 gallons of rainwater (8.3 Lbs. per gallon of water equates to over 3000 Lbs. of water, a ton and one-half ton). Spread over the foundation perimeter of 140' for a modest 1200 Sq. foot ranch home, every linear foot of the foundation perimeter then is subject to dealing with an over 21 lbs. of water or hydrostatic pressure. 

After the water is absorbed into the soil, pressure and weight is still a factor, until the ground is able to dissipate the water naturally. This is water shed from the roof and does not take into account the water being collected in the ground while it is raining.  It is important to have your downspouts directed away from your foundation and preferably in an area that slopes away from your residence. ABD can provide options to manage rainwater runoff and also manage low spots in your yard where rainwater collects. See more on exterior water management.