This is intended to be the final "old client" release before SegWit. This wallet update deprecates and removes the Stealth Addresses function since it will not be available in the next wallet version. Additionally, this release implements lowS signatures which is required for BIP66. Finally, it updates the default transaction fee structure so that transactions from this wallet will be propagated by the new wallet once it is released.
Whilst this update is not mandatory, it is highly recommended as it will take longer and longer for pre v0.10.1 wallets' transactions to be included in a block as more of the network upgrades to the new SegWit wallet.