Dylan16807 is correct. You can't forge transactions. You can create an alternative history, but each transaction still requires holding the necessary private key to sign the transaction. If you're trying to make up new transactions out of whole cloth, you can only do so if they're transactions that you could have otherwise legitimately created.
What the 51% attack lets you do is remove transactions, which lets you double-spend. And of course there may be other benefits too. For example, if someone is using the blockchain for purposes other than sending money to people (e.g. notarizing a document, proxy transactions that represent movement of physical goods, etc) then removing transactions may be beneficial without double-spending.
What the 51% attack lets you do is remove transactions, which lets you double-spend. And of course there may be other benefits too. For example, if someone is using the blockchain for purposes other than sending money to people (e.g. notarizing a document, proxy transactions that represent movement of physical goods, etc) then removing transactions may be beneficial without double-spending.