为什么要加 delete onEnterFrame ;

请看这段代码
loadMovieNum ( "02.swf" , 1 );
onEnterFrame = function () {
if ( _level1 ) {
with ( _level1 ) {
_x = 50 ;
_y = 100 ;
}
delete onEnterFrame ;
}
};

其中 delete onEnterFrame ; 删掉后效果是一样的，为什么要加这一局呢

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