Coincidentally, it happens to be the day the Jews were expelled from England in 1290 and from Spain in 1492 and the day deportations began from the Warsaw Ghetto in 1942.
Since most Jews today, with the exception of some Orthodox, are not looking for a return to temple practices (which include animal sacrifice), the symbolism of the fast day has, in some places taken on new meaning.
I love ritual. It is one of the things that most connects me to my Jewish community. That there is a proscribed time and a place for everything, for mourning and for giving to the poor, for getting drunk and for working toward a better life. These are all things that we are reminded to do throughout the year (not that we can't get a little drunk or give to the poor at other times...).
Anyway, I need Tisha b'Av, a time and a ritual to mourn destruction. I think we all do otherwise our time to mourn the losses of our past get filtered a little into our everyday lives, feeling like we never have a proper time to mourn anymore.
I'm not saying that we should not or do not feel our grief and sadness the rest of the year, but we have no place to say, "On this day, I mourn. I mourn my childhood, destroyed at the hands of someone else. I mourn the loss of a generation to HIV. I mourn for the deaths of children, and women and men throughout the world due to violence."
I believe it to be so important. And so, on this day, I mourn.