Impact assessments build on knowledge about causalities. For some effects, though, this knowledge is still very limited, which makes it challenging to quantify impacts. Ignoring these effects may, however, lead to distorted results: Just because we cannot properly model an effect, does not mean that it doesn't exist! 

The importance of such effects can be illustrated by a simple (hypothetical) example:
If an assessment were to analyse the impacts of afforesting all agricultural areas in Europe, ignoring leakage effects and indirect land use changes would lead to assessment results showing overwhelmingly positive effects for climate change mitigation. However, it is more realistic to assume that such a scenario would also lead to strongly increased food imports, a surge in food prices and a conversion of natural areas in other countries to farmland. This conversion would then partially or fully offset any positive effects on the global climate. Assessments that ignore these effects may therefore produce misleading assessment results.