Behaviors of Site Columns of the type Choice

I created a site column in Central Admin. Then I added the site column to a custom list. I was able to modify the choice options of the site column instance. This can be a good thing but not sure if it defeats the purpose of a site column. I found that if I want to reset the local instance back to the values of the main site column definition, then I can go back to Central Admin and update the site column, and the updates will override the modified instance that I had created in my list.