The main reason to think about the long term effects of oil drilling is that these are effects that are generally very difficult to reverse. If we drill for oil without adequately considering the sorts of problems that might arise due to drilling, we might end up bringing about these negative effects at which point it will be very difficult to reverse them. We must also think about the long term benefits, of course, and ensure that we balance them against the costs.
Many of the impacts of oil drilling affect us in the long term. The most obvious of these is global warming. The more oil we use, the more likely we are to cause climate change. As far as we know, it is very difficult to reverse climate change once it has started. A second impact is the environmental damage that can be done to areas that are affected by oil spills. When ecosystems are destroyed by an oil spill it can take them a very long time to rebound.
Because the long term effects of drilling can be so hard to reverse, we must take them into account before we start drilling.