Common subexpression elimination is a common way to optimize any programming code. Doing unnecessary selector is expensive. Doing something like this:

jQuery('.class').each(function(){
	jQuery(this).html();
	jQuery(this).find('div').each(function(){
		//etc.
	});
});

which required many selector it is best to use 1 instead since we are repeating ourselves and doing some redundant selector.

jQuery('.class').each(function(){
	var obj = jQuery(this);
	obj.html();
	obj.find('div').each(function(){
		//etc.
	});
});

This is something that we often see in many plugin. Many plugin contains unnecessary selector which degrade the perform of the plugin little by little.