The flu is an interesting beast. Let's say you are a flu virus that has just landed in someone's lung tissue. What's your next move? You had best get inside of a cell, and quick! You see, the human body has agents called white blood cells out looking for you.

Ok, now what's your next move? Well, you want to reproduce. Really, that's all you want to do. You don't care if you reproduce in this host or another host, so long as you get the chance to make many, many offspring. The problem is, eventually this host's immune system will catch on and eradicate you. So, you had best make plans to find a new host.

Now you have two choices:

1. You could reproduce rapidly. This will allow you to generate the greatest number of viral particles before the host immune system catches on. Unfortunately, as you reproduce your host will become ill. They will be less mobile and more likely to be quarantined, so you might end up coming in contact with fewer potential new hosts.

2. You could reproduce at a measured pace. You'll end up with fewer offspring total before the immune system knocks you out, but if you can leave your host mostly healthy, then they are more likely to bring you in contact with more potential new hosts.

As for why you are contagious before the onset of symptoms, that is due to another rather interesting feature of the flu. You see, the flu can cause damage in a variety of different ways. Obviously, taking over the machinery of a cell to make new virus particles is not good for the cell or a tissue. However, that's not the most damaging effect that flu can have.

For particularly fatal cases of the flu, what happens is that so many viral particles are produced so rapidly that the body's immune system goes into overdrive. Never underestimate the power of the human immune system, especially the power to kill humans! This is why the Spanish flu, the Swine flu, and H5N1 are most devastating to relatively young, relatively healthy individuals (their immune systems are the strongest).

So, the flu symptoms that result from the body's own immune system causing damage will only occur (obviously) once the immune system has already engaged and begun neutralizing the virus. Hence, this is why flu is typically only communicative before the onset of symptoms.

Edit: I should add that despite this interesting feature of the flu, almost all diseases are primarily communicable before the onset of symptoms. This has as much to do with the immune system as it does with the fact that generalized symptoms won't set in until the buggers have already been reproducing for a while.