Ancient Greece had few horses (except in the plains of Thessal
onia), so infantry became the dominant part of land warfare.
The Greek city-states developed heavy infantry and an almost ritualised form of warfare that required the frontal confrontation of two battle lines of heavy infantrymen.
This changed profoundly when Athens (+allies) and Lacedaemonians (Spartans, + allies) faced each other. The Athenians were superior at sea and inferior on land and refused a decisive land battle. This long (one generation!) Peloponnesian War became a civilisation-shattering experience that pretty much ended the phase of cultural prosperity and advances of Greece. On the other hand, it began a time of military innovations in the Hellenic World.
A generation later the Lacedaemonians faced off against the numerically inferior Thebans in the Battle of Leuctra, and the apparently genius Theban general Epaminondas finally introduced serious tactics into Hellennic warfare; the famous oblique order that overwhelmed one wing of the enemy and thus turned the enemy's line. It was a very early application of the Schwerpunkt principle.
Up to that time numbers of men and their quality as hoplites (including their equipment) had been the most decisive factor.
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Hoplite equipment evolved over time. One trend (attributed to the general Iphicrates) took place after the Peloponnesian War and included longer spears, lighter armour (more mobility as sword fighter) and can be considered to have been a reaction to the rise of light infantry (skirmishers) such as the Peltast javelineers.
Most literature about Iphicrates-style hoplites and later Macedonian heavy infantry with even longer Sarissa spears appears to focus on the spear length. I suspect that the Sarissa (r)evolution was overshadowed by the much more drastic changes favoured by Iphicrates. Iphicrates probably deserves to be remembered as the first infantry equipment revolutionary (leaving Marius far behind).
First, an example that helps to understand my point: The German techno music festival "Love Parade" experienced a disaster a few weeks ago when 21 people died and about 500 were injured. The cause? The masses were pressing towards a stair. This pressure sufficed to kill and badly injure people. The deaths occurred because of deadly pressure on the upper bodies.
This scenario is quite similar to how Hellenic battles worked before the spears grew very, very long. The clash was at least as much about pushing as about stabbing with spears and swords (the aparently preferred Greek sword, the kopis, was rather a slashing than a stabbing weapon).
The forward ranks needed a stable bronze cuirass. This did probably provide more crucial protection against the ranks behind its wearer than against the enemy (who was usually behind two shields).
Greek phalanx warfare was almost always decided when one phalanx broke. There was no real exploitation phase, fleeing warrior-citizens were not cut down in a pursuit (remember, few horses!). Losses per battle were therefore low both absolute and in per cent. This made frequent wars over tiny conflicts sustainable for the city-states.
Now think about it; you need no armour on your back to protect against the enemy in such a kind of combat. Greek bronze cuirasses were still a two-part body armour, though: Front and back part. No weight- (and cost-) saving half-cuirasses apparently.
That's because the body armour was apparently very much about protecting against the hoplite's fellow warrior-citizens in ranks behind him. It wasn't only (or mostly) meant to protect against the enemy. That's also why other forms of body armour such as scale armour or composite armour (= less able to protect against pressure) were not important for the forward ranks of the hoplites.
The "pressure" component of phalanx tactics was maximised with the oblique order when the breakthrough wing of the Battle of Leuctra was 50 ranks deep. (Btw, it was led by an elite group, the Sacred Band of Thebes. This is interesting if not funny with DADT in mind.) There can also be no doubt that the psychological effect of facing a formation many times as deep as yours was of great importance, too.
Iphicrates changed this by dropping the bronze cuirass. He preferred the lighter, composite body armour of textile and glue (linothorax), and apparently so also for the front ranks. This seemed to mostly rule out the pushing and to emphasize the stabbing with spears.
The next step was even longer spears and serious training - the Macedonian Phalanx.
In short: I believe that the move towards longer spears was about much more than mere out-ranging the enemy.
It was a radical shift away from a pushing & stabbing tactic to a stabbing tactic. It made extremely deep formations unnecessary and thus enabled very wide (and again relatively thin) battle lines.
This in turn enabled the Hellennic heavy infantry model to be effective on wide plains (Alexander's conquest of Persia) instead of only on the restricting terrain of Greece (where both flanks were secured against heavy infantry formations by hills if not mountains or the sea).
This was something that seems to be missing in many books and webpages about classical Greek warfare. The authors never seemed to connect the dots.
I cannot guarantee for the correctness of my interpretation, of course. It all just fits nicely together if you look at it from this angle.