Many organizations tell me that they hear from their participants all the time telling them how much the program changed their lives. Understandably, those experiences matter a lot to organizations and they want to capture those experiences in their evaluations.
Recently I heard a podcast that perfectly captured the risks in relying too heavily on those kinds of reports. There are two related issues here. The first is that while your program may have changed the lives of a few participants, your evaluation is looking to determine whether you made a difference for the majority of participants. The second is that you are most likely to hear from participants who feel very strongly about your program, and less likely to hear from those who were less affected by it. An evaluation will ensure that you are hearing from a representative sample of participants (or all participants) and not just a small group that may be biased in a particular direction.
An evaluation plan can ensure you capture both qualitative and quantitative measures of your impact in a way that accurately reflects the experiences of your participants.