How can i distinguish the difference on functions between centralized db and permissioned blockchain?
Well the most glaring difference is that you can't alter the data on a blockchain, while the same isn't true for a database.
what is the benefits of permissioned blockchain?
The only obvious one I can see is immutability. Permissioned blockchains could also be more distributed thanks to its nature, so it may make it easier to protect from data loss. Other than those, I guess it really just boils down to taking advantage of their key differences.