Although each technique is quite simple – it isn't necessarily easy. It's like watching a movie or reading a book. When you go through it the first time – you get a good general idea of what's going on and what happened and how it started and ended. Though, to get finer details, you may need to watch it again, and again.

Each time you watch/read that story, you'll pick up more details you didn't really notice before.

It's pretty much the same principle here, with your Taekwondo technique. You may think you know it, and maybe there's a new aspect to it you just weren't aware of before. When your instructor goes through it again, you may hear something from a different point-of-view. You may understand it more clearly from a physics angle, etc. and so on.

Eventually, (depending on how you were first taught and how YOU understood that first lesson) your more recent effort will be vastly different from your first effort; and sometimes it might pay to just forget that ‘first time’.