Q. Why fast?
A. I knew a lady in London who told me that the minister of her Liberal congregation used to say that without his morning cup of tea he couldn’t get through his Yom Kippur sermon.
I don’t know what the full facts were; I do know that he was regarded as a very fine preacher. Maybe he gave himself a dispensation to have a cup of tea, maybe he didn’t – but unless you are really ill, the halachah cannot allow you to break the fast by eating and drinking.
Spiritually it does you more good to fast than to eat and drink. You need to feel physically deprived and psychologically on edge for the day to work on you.
You also need a reminder than though you can resume normal eating and drinking after the fast, many people are hungry and homeless all the time, and you and I need to decide to do more for them to ensure that every day is no longer a Yom Kippur for them.