Because the lay practitioner is exposed to conventional reality more than monastics, making that division is more important in the practice for protection. Conventional reality is samsara as it has been adapted by society through laws and institutions, for example sexual desire into the custom of marriage. Every time a practitioner goes into public they become mentally constrained by the forces of conventional reality, and need to note the difference between the mind in solitude and in social life. These are human developments which have a certain validity, yet remain of less authority than ultimate reality or nibbana. That’s why the arahant goes along with conventions but is not attached to them, understanding they refer to managing the body. Every time samsara or ‘the ordinary uninstructed person’ is mentioned in the suttas it incorporates conventional reality. That person has no other reality than convention, and a practitioner has to be aware of that, doing so avoids suffering. That’s also what it means in the suttas by "going against the flow,’ and samsara is a definite current so once recognized, practice can never be passive. From the beginning your practice is classified as ultimate reality, and a division opens the door to insight.