Microsoft Timezone User Interface

Hello, I am relatively new to this discussion, and I am interested to know your thoughts on the following. The Microsoft Windows Time Zone User Interface allows you to use (with a check box) daylight savings in all time zones where applicable. I was wondering as to why they made that a user modifiable option? They could for example have had two Eastern Standard Time (EST) pulled down options, one using daylight savings, and the other not using daylight savings (for same parts of Indiana). Is it for flexibility? or .....? Thank you. Srini