The planning process considers economic, social and environmental issues up to a generation into the future. Buildings can last much longer than this, so it is important to make sure they are in the right place and fit for the future. Failures to think through these issues can be expensive to put right. For example, new health centres built on the edge of town away from public transport and without safe access to walk or cycle to can be costly to run because of specialised transport needed to get people there.
Planning is a whole subject in itself and our sister site, Planning Help, provides lots more information.