Nowadays, we have a habit of just throwing anything into the wash whenever we feel like it as we have all of the machinery that allows us to do just that. However, in the past it was an expensive thing to wash your clothes which is why they came up with a rather clever way of dealing with shirts in order to save money.
With a shirt, the collar is always going to be the one area that is exposed to the biggest amount of dirt. As a result, the collar looks filthy way before any other parts of the shirt, so when washing your clothes was expensive, it was decided that it would be a waste of time, and money, to launder the entire shirt every single time the collar was dirty.
So, what did they do?
They decided that the collars of the shirts should all be able to be taken off all on their own. By doing this, it meant that the collar could be washed by itself and save you washing the rest of the shirt when it was certainly still able to be worn. It cut time and also saved money while people were still able to look clean, which was always very important to them. However, when washing your clothes became less expensive, the collar found itself being attached to the shirt once more.