Rain drains are what your downspouts connect to.
In new construction they may be plumbed to the storm sewer, a dry well, a storm water retention pond or some other type of vent.
If rain drain discharges get plugged with debris and age, it is unlikely that you will be able to… go back into the rain drain discharge system, buried underground and not marked, to the storm sewer, even if you could afford it.
The construction of hand excavated french drain groundwater removal systems, containing new rain drain discharge lines in solid pipe, is usually advised.
Never flush gutters into your rain drains when you are cleaning them. This only makes them plug faster.
Water that overflows your gutters saturates the foundation walls causing groundwater to run into the basement and crawlspace areas.
Water spilling over your gutters, due to clogged rain drain discharge pipes, can undermine foundation footings and cause damage to the foundation walls, both inside and out.
Water that overflows the roofs rain gutters can saturate the foundation wall and cause a leaching of the lime onto the interior and exterior of the walls, causing cracks to powder and crumble with time.
Mold is another factor that also comes into play in time.
Water overflowing your gutter system and rain drain discharges can flow over to your neighbors lot as well, and cause groundwater problems in their crawlspace and basement.
This can lead to hurt feelings and spirited debate with your neighbors at best, and litigation and bad feelings at worst.
Gutter helmets and gutter guards can cause the heavy rains to just shoot over the gutters, and act as if your home has no gutters at all.
While in moderate to slow rains these systems wick the water under the lip of the guard as the are designed to do, in heavy rains they do not do it well enough, or at all, in my opinion.
The number and type of roofing layers that have been applied, and the style of homes roof line, also makes this more or less of a problem.