just got this point from :
https://bitcoincore.org/en/2016/01/26/segwit-benefits/Segwit prevents third-party and scriptSig malleability by allowing Bitcoin users to move the malleable parts of the transaction into the transaction witness, and segregating that witness so that changes to the witness does not affect calculation of the txid.
and also some important points
Who benefits?
- Wallet authors tracking spent bitcoins: it’s easiest to monitor the status of your own outgoing transactions by simply looking them up by txid. But in a system with third-party malleability, wallets must implement extra code to be able to deal with changed txids.
- Anyone spending unconfirmed transactions: if Alice pays Bob in transaction 1, Bob uses that payment to pay Charlie in transaction 2, and then Alice’s payment gets malleated and confirmed with a different txid, then transaction 2 is now invalid and Charlie has not been paid. If Bob is trustworthy, he will reissue the payment to Charlie; but if he isn’t, he can simply keep those bitcoins for himself.
- The Lightning Network: with third-party and scriptSig malleability fixed, the Lightning Network is less complicated to implement and significantly more efficient in its use of space on the blockchain. With scriptSig malleability removed, it also becomes possible to run lightweight Lightning clients that outsource monitoring the blockchain, instead of each Lightning client needing to also be a full Bitcoin node.
- Anyone using the block chain: smart contracts today, such as micropayment channels, and anticipated new smart contracts, become less complicated to design, understand, and monitor.
