Oftentimes we are tempted to just collect data for the beneficiaries of our programme.
However, this is short-sighted. I talk from personal experience as I learnt the consequence of this mistake in the past.
At some point an evaluation or another study will be done and then the brilliant idea hits us, 'it would be so nice to compare the results of persons who participated in our programme versus persons who did not benefit from our initiative'.
However, that idea may never see the light of day as it may be too time consuming to find a comparison group. All this could have been easier if the contact details of everyone (not just the selected persons) were recorded when they applied to be part of our programme.
Even if a comparison group is never needed, beneficiaries may drop out of the programme and may need to be replaced. It is so much easier to do this replacement if the names of prospective candidates are already in your database.