If you upload your ad creative on Tiktok user account, you could see how the creative performs organically before boosting it. More so, comments, likes, shares, follows and other engagements made on that post remains on the post. Promoting your Tiktok posts means that while you are running ads, you are essentially promoting your Tiktok account at the same time. On the other hand, uploading a video directly on Tiktok ad manager and promoting it, means you can start running ads much more quickly even if you don't have a Tiktok account!