Why do we use $A.enqueueAction(action) in Salesforce?

$A.enqueueAction(action) sends the request the server. More precisely, it adds the call to the queue of asynchronous server calls. That queue is an optimization feature of Lightning.

({
doInit : function(component, event) {
    var action = component.get("c.findAll");
    action.setCallback(this, function(a) {
        component.set("v.contacts", a.getReturnValue());
    });
    $A.enqueueAction(action);
}
})