Protection, protection, protection.
When a house is built to correct code, it is placed on a slight slope that will help guide runoff away from your foundation. If rain flows off your roof with no gutter system in place, the water can cause massive erosion, washing away more soil every time that it rains. Your carefully sloped landscape can now be wearing down, causing runoff to flow towards your home instead of away from it.
A lot of homeowners enjoy the looks of flower beds around their home. If you do now have a proper guttering system, your garden can literally be washed away from the soil erosion.
Rainwater can carry many things with it off your roof. Leaves, dirt, and tiny asphalt particles can run down your siding, leaving an unpleasant site. Over time, rainwater can begin to rot your siding and especially if it's made of wood. Of course, rotten siding isn't very appealing but it can also create holes that will allow pests into your house. With enough water seeping through your siding, this can affect your home's structural integrity and can cost you money for damages down the road.