Why my tokens wiped out on devnet?
Currently SUI only have devnet, mainly for test & development purpose. And for devnet itself may experience break changes, the hole blockchain may wiped out by SUI official when its needed.
Once the whole blockchain being wiped out, your assets will disappear and not longer exists, but your wallet itself will still be there. You can follow this doc How to get SUI tokens? to get some SUI tokens at the beginning.
After the SUI goes to mainnet, then your wallet will never be wiped out, then you can trade your assets safely on SUI blockchain. We expect SUI mainnet will goes online in next year(2023), be patient.