Saturday is a clinical day, not a weekend
A Saturday clinic concentrates a full session's worth of patients into a few hours, often with a higher proportion of families and walk-in appointments than a weekday morning. The waiting room fills and stays full, the washroom takes heavy use, consulting rooms turn over repeatedly and the waste generated is comparable to any other clinic day. None of that changes because it happens on a weekend.
Leaving that until Monday evening means the premises sit in a used clinical state for around sixty hours. For a general office that would be untidy; for a practice it is a different order of problem, and it is one that is entirely visible to the patients who arrive first thing on Monday. A short Saturday attendance addresses it at modest cost.
