Quote from: Akiyoshi on March 30, 2018, 09:28:16 AM

why the system using blockchain need them, PoW,PoS?

Because you want to have "cryptographical" proof of your transactions being valid (or rather "work") and those of other people, which is what PoW accomplishes.
A more detailed answer can be found here, https://bitcoin.stackexchange.com/a/51296

With POS you have collateral backing up your transaction ( or rather the blockchain), which also creates consensus and thus a way for users to trust the system. ( Since that is the ultimate goal ( being able to place trust in a decentralized system))

https://ethereum.stackexchange.com/a/141