Take the practice of Shradham, which we do for the dead. It is just a simple logic of remembering the dead at least once in a year. In our day to day life, we do not have time or inclination to think of the dead all the time, or for that matter it is not practicable. But once in a year one need to remember the dead. In Christianity they go to the burial ground and place few flowers on the grave of the dead. Since in most of the Hindu groups, it is cremation, we need to remember them in the way of Shradham.
In the case of isolation during menstrual period, Manu has prescribed isolation for women with women dignity in mind. Manu wrote the book, more than few thousands of years ago, and at that time the society had different way of living. The science was not advanced as it is today. There were no sanitary napkins available at that time. Even clothes were of very basic needs. And think of the dignity of the woman if there was no isolation!!
Another practice we say is of not entering the household with out taking a bath after a shave or haircut. It has no religious value, but of hygiene and health concern in it. There were not saloons and the places where they used to have haircut or shave used to be on the banks of river or in the backyard of the house. The barber used to have basic instrument to do his job and at the end of it the hair used to be all over the person. By entering the house hold with out having a bath would endanger the members of the family to food poisoning!!
