Yes I know it was an important battle and as far the saying goes "he lost the fight but won the war" . But he was in one known fight with 300 people (well with many others with him but only 300 agoge trained soldiers) and he lost that fight (as he probably expected).
What was the reason he was praised so much and still got to become famous and even got a movie made after his lost after thousands of years and have monuments and sculpture after him. Was the sole reason is bravery of fighting against a huge army with a tiny fraction of enemy type of army and gave the greek armies time to unite and fight back?
It's a very interesting question, for his fame tells us much about how values, ideology and tradition shaped the collective memory of the Greeks (and the Spartans especially). The best way to undestand the issue is to read Herodotus' account of the battle: it clearly follows the Spartan tradition about the battle, and contributed in creating Leonidas' myth.
First the facts: in 480 BC the Persian king Xerxes attempted to subdue Greece in a great expetition. Some Greek cities, leadad by Sparta, opposed the Persian invasion and organized resistance. The Greek strategy was of defending passes, easier to defend with fewer soldiers. They ultimately chose the Thermopylae: the Greek force had circa 4000 heavy-armed hoplites (and other light armed, we don't know how many), among which the 300 Spartans. They resisted a few days, until the pass was bypassed by the Persians. While most of the Greek soldiers retired south, the 300 Spartans, 700 Thespians, and (unwillingly) 400 Thebans stood ground and were (almost) all killed (the Thebans apparently surrendered).
It's not clear why, in the reality, Leonidas chose to stay and be killed: for pure bravery? for pride? In order to let the other Greeks retire safely? Was he just a bad strategist? All of these reasons have been proposted by ancient and modern historians. Let's skip the real reasons and let's see how the tradition preserved by Herodotus presents the Thermopylae event.
Even if there are a lot of other troops, the heroes are the Spartans. This is not only because they were the most skilled hoplites, but also because they had the most prestige. Moreover, it's clear that the accounts we have of the battle derive from a Spartan propaganda, which aimed to show that Thermopylae was a Spartan feat.
The Greeks would have been undefeated, if not for Ephialtes' treachery. This is probably an exaggeration: the 4000 hoplites could hope to hold the ground a few days, maybe waiting for reinforcements. Yet the story is presented as if the Greek (Spartan) military value was so great it could have stopped the enemy, no matter how big they were.
Sparta's νόμος δεσπότης. Spartan exile king Demaratus tried to explain to Xerxes, Herodotus tells us, that the Spartans are greatest warriors because they obey to a supreme law: never they do flee, but stand in place and fight, winning or dying in the attempt. This is the sharpest proclamation of hoplite ethics. Quasi as a scientifical demonstration, Leonidas' 300 follow the rule to the extreme, showing the Persian-and the other Greeks, the true value of Sparta.
The false prophecy. Herodotus cites a prophecy, clearly created after the battle, which said that "either a Spartan king would die, or Sparta would be destroyed". No matter how much the prophecy was believed by the Greeks, it created a further reason for the sacrifice.
Briefly, Leonidas' death was seen, by Greek and Spartan propaganda after the victory, as a symbol of hoplite value and Spartan strength: the demonstration that Greek and Spartans were tough and unyielding; besides, his defeat was due to treachery and couldn't lessen his glory. The defeat itself was then considered a sacrifice to ultimately save Sparta and Greece. Such a powerful image easily gain immortal fame, don't you think?