Reality checks serve multiple purposes. They raise your awareness of your state. get you into the habit of questioning your reality, and confirm whether you're dreaming or not. All essential for lucid dreaming.
You may think you know you're awake, but the truth is you can never know 100%. You'd be amazed how convincing dreams can be, I've had dreams which were identical to waking life, I could've sworn I was awake, but I was not. So you can never be sure you're awake. You'll see that for yourself some day if you have an extremely convincing dream.