Regarding this practice, unfortunately, I was disadvantaged.
I was introduced to enemas in a hospital when I was four years old. I have explained this introduction in more detail in some of my other posts, so I am not going to reiterate, but feel free to read my posts if you like.
With reference to this question it was not the standard practice back then for the nurses in that hospital to insert a well lubricated finger inside the patient's anus before giving them an enema. It was never done to me and I did not see it done to other patients.
As I have also previously stated, I work in a hospital as a nurse assistant and I have occasionally been called on to assisted with the administration of a fleet enema to a patient, but the bottle nozzle is always lubricated by the nurse prior to insertion into the patient's anus. (There are laws regarding digital penetration and consent must always be obtained).
So whilst I am not denying that this practice of inserting a lubricated finger before an enema was and is still performed and greeted with appreciation, I would speculate that it probably occurred more so in a non-clinical environment like in a private and/or domestic setting when your mother/grandmother/aunt/girlfriend (or boyfriend) gave/gives you an enema.
FYI, I self-administer, and I do like to insert a well lubricated finger inside my anus before I have an enema - it certainly makes everything more pleasant.