For me, the reasons I want to know of everyone living in the home (AND have them properly reflected on the lease) is for legal, liability, insurance and compliance purposes. First, I do not want someone (an adult) to be able to claim residency rights if I do not have that person's information and have run credit, background and employment / income verification on them. Maybe they do not meet my criteria for residency, such as a recently convicted felon or smoker. I also want to make sure the property remains within zoned occupancy limits, and that 2 or 3 families are not living in a single-family home, or just too many people in general. As the number of person's living in a home will impact the wear and tear, and therefore potentially the rent that I must charge and security deposit requirement (all per federal/state housing laws of course). Also, I need to know who all is present in the home should there be a catastrophic event, like the place burns down.
I make it an automated practice to send reminders to tenants at certain points in their lease term (that require a response) to inform us of any changes to their household makeup as reflected on the lease - and that failure to do so is a lease violation that could result in lease NON-renewal or termination / eviction filing.