The idea is that you can be guilty even though you were not aware of X or had taken reasonable steps as to preventing X. Thus, it encourages you to go above and beyond in order to ascertain X is true or that X does not happen. This is particularly important with regard to public safety offences.
There are also cases where it's difficult to prove that D had the mens rea and so strict liability is adopted. For example, if D is a company, it is impossible to show D had a particular mens rea.