Unique TikTok usernames help build a recognizable brand that lets you stand out on the platform. This is especially important if you want to grow a following and eventually monetize your account. It's also a great way to make sure your content is attributed to you and not someone else.
It's also a good idea to be consistent with your username across social media platforms. That way, when people search for you outside of TikTok, it's easier to find your other accounts. Actress and singer Selena Gomez is "@selenagomez" on TikTok, Instagram, and Twitter.