You Unlock All the Tiers You Already Completed
Have you been playing the free version of Fortnite Battle Royale, and just learned about the Battle Pass?
If you buy the Season 3 Battle Pass all of the tiers you unlocked already are retroactively applied so you will get all of the special rewards without needing to start over or level up from scratch.
This is great news, especially late in Season 3. It's also helpful if you've been playing but couldn't decide if you wanted to spend the $10 on the Battle Pass. If you pair this up with the Battle Pass Bundle you also unlock the next 25 levels automatically which can be helpful late in the season.
Look at your in game level and see what rewards you would get if you upgraded to the Battle Pass and decide if it is worth it.