On this page of the guide to Crash Bandicoot 4 you will find the answer to the question - what are the rewards for completing the additional levels?
The main reward is unlocking new skins. However, in order to get them you must max-out all 21 levels by collecting 100% crates in them.
Each level can be easily rated based on three medal levels.
- First - for just completing the level;
- Second - for getting 90% of the crates;
- Third - for getting all the crates.
Remember that there are no secrets and collectibles to find in these levels. All you need to do is reach the end of the level.