It seems to me like it required a lot of organisation and expense to arrange a mass battle on the field. From what I've heard, most battles were skirmishes, sieges or raids for pillaging, because that's what generally won a war.
Why then, were there pitched battles? What examples can be drawn up for why this was, at times, necessary?
For Classical Greece, there's a couple of reasons. The main one is that the city-states had no professional army and no money to create one. Their military forces were literally the population in arms. Since most people could not afford to leave their farms and shops for very long, campaigns had to be short and decisive. Pitched battle was better suited to this purpose than skirmishing, raiding and siege.
Of course the Greeks strained against these limitations, knowing that prolonged campaigns by mercenary or specialist expeditionary forces could entail less risk and bring more profit to them. Even so, the ideal of a quick (and cheap) decision remained attractive, not least because it was the most straightforward way of imposing one's will on the enemy. An army abroad would find more safety in a direct approach than in a slow and tentative campaign that allowed the enemy to take initiatives of his own. Meanwhile, an invader would win a moral victory if he got to ravage enemy land without encountering resistance; men who watched their farms burn from the safety of the city wall would generally be spoiling for a fight. For these reasons, even the forces despatched for longer campaigns often ended up fighting pitched battles too.
That said, the Greeks were very careful to avoid pitched battle if they felt that they would be fighting at any kind of a disadvantage. They wouldn't engage unless they were confident of their numbers and skill, and ideally they would look for some suitable ground and try to take the enemy by surprise. Even if there was a moral imperative to fight a pitched battle, everything was done to make sure that battle would not be fair.
Because of the risks involved, pitched battles were actually not common. Considering the fact that "ancient history" covers a period of several thousand years, and even the age called Classical Greece lasted a century and a half, we actually know very few pitched battles, and even less are actually widely known among a general audience. The ancients agreed that the bloodiest and most protracted conflicts of their past were the Peloponnesian War and the First Punic War; can you name a pitched battle from either?