Anybody want to tackle that question?
Me! Me! Pick Me!
Maybe because he was concerned about the safety of his neighborhood. Being part of the Neighborhood Watch, about which Sanford Police say: "Training provided by law enforcement agencies to Neighborhood Watch organizations stresses non-contact surveillance of suspicious situations and notifying police of those situations so that law enforcement can respond and take control of the situation."
"Surveillance" would be "maintaining sight," in case you didn't make that connection. And he did call police, in case you missed that, too.
Frankly, I am a little tired of the idea that Zimmerman leaving his car was even unreasonable, much less illegal. One might argue it was poor judgment, as Zimmerman might (in fact, did) put himself at risk, but there is nothing illegal or threatening about walking around your own neighborhood. Or even asking a stranger what he's doing there. It certainly doesn't give that stranger a right to punch you.
Now we don't know for a fact that it happened exactly the way Zimmerman says, but his leaving the car and trying to keep an eye on Martin is certainly no reason to doubt it.