No one really needs a special day to tell someone how much they love them. But then again if I were to get dressed up and take my wife out to dinner, buy her flowers, go see a live performance and so on, it wouldn't be very special at all would it? Even the best steak possible gets boring if you have it every day.
This is why it's a special day. You don't have to spend a cent on it either. It's the expression not the value of the expression that matters.
One year we had to spent a few days apart and we hate to be a part even for over night let alone days. It was Valentines day and when I got to the hotel and opened my bag I found all these little notes in the shape of hearts that said things like "I love you" and "here is a kiss" and "here is a hug". She blows my mind that way.
It's all about taking the time to focus on each other for a day. There are times when you can do that all throughout the year, but having a day for it only clears the decks so that you have an excuse not to sit eating dinner in front of the tube, have a couple drinks and go to bed so you can get up and earn your keep the next day.