Why do i need to post a 100% speedrun playthrough before requesting a game? Just curious as to why

You don't need to do a 100% speedrun to submit a game. You need to do a speedrun that shows much/most of the game, both to show the details of the game as mentioned by @Colors and to help demonstrate that you have an understanding of what speedrunning the game entails. After all, you will be a moderator of the leaderboard if the game gets accepted.

When submitting a game with a less than 100% speedrun, I like to give a brief explanation of what was left out of the run in the "additional notes" section of the game request. For example, when I submitted this game I explained that unlocking the secret boss requires fighting two additional secret enemies who are visible during the any% run, and that only the secret boss isn't shown in the vid.