The primary benefit of Diceware over a "random" string of characters is that it is easy to remember and truly random. With a password manager you don't need to remember the password and it will be generated truly randomly. A string of 11 random alphanumeric charatcers has more entropy than a 5 word diceware passphrase with the added benefit that it is less to type if you need to do so manually. But diceware can be a good idea for creating the master password for your password manager and if you do that you should probably use a 10 word passphrase rather than 5.