Why is King Leonidas of Sparta important now?

The answer to this question is mostly one of opinion. Not much is known about the actual King Leonidas as he exists as both a legendary and historical figure. Most of what we know about him comes from the ancient historian Herodotus, who is well known to have exaggerated facts, or even outright invented them. Leonidas, I think, is a great case study in human nature, even in a modern sense. Although ancient Sparta existed in a time and place far removed from today's world, in many ways it was surprisingly modern.
For instance, at the time of King Leonidas, Sparta ensured access to public education for all its citizens, much the way many nations do today. Sparta also had a social welfare system in which land was granted to all Spartans so that they could farm it and support themselves. Sparta also provided more autonomy to its women than any other Greek city at the time. These can all be viewed as examples of how to build a more inclusive and supportive society today.
As for King Leonidas, it is important to note his conscious self-sacrifice. He can be seen today as a hero who gave his life for the greater good of his nation. He went to war against the Persians in order to defend his home. It was not an aggressive attack, but rather a defensive action. Herodotus makes it clear that Leonidas knew he likely would not survive the Battle of Thermopylae. Herodotus writes that on the morning of the battle, Leonidas "bade his soldiers eat their breakfast as if they were to eat their dinner in the other world." Leonidas can be seen as a martyr who put his nation's survival above his own. Today's leaders can look to him as someone who rules (or in this case fights) not for his own glory, but for the greater good of the people who look up to him as a leader.
http://www.sjsu.edu/people/james.lindahl/courses/Hum1A/s3/Thermopolyaesm.pdf

https://www.ancient.eu/sparta/