Being responsible for evaluating the social impact of a charity does not always make you popular. One definition of an evaluator is a someone who shows up on the field of battle after the fighting is all done, seeks out the survivors, and shoots them!
Assessing the impact of an organisation takes up resources, requires staff to collect and collate data when they would invariably rather be doing something else, and the results are all too often disappointing to committed and optimistic staff and trustees—a sobering reminder of how hard it is to bring about change.
Done well, monitoring and evaluation can provide useful insights into what is working and what is not, it can inform management decisions about how to divide up valuable resources, please funders, and influence other stakeholders and policy. But done poorly, you risk producing large amounts of questionable data, spending hours analysing it, only to find you cannot draw any meaningful conclusions.