Strings of everyday words are better than the passwords most people choose, they're memorable, and they're often good enough from a practical perspective. But if you're using a password manager and don't have to remember passwords, you might as well use truly random passwords, which have more entropy.
I disagree. I use a 1Password, and I used to do the "totally random string of letters, symbols and digits", but I've dropped it for the "four or five random english words" alternative for all new passwords.
There are a couple of situations where having these symbol strings is really inconvenient. For instance, reading a password out loud to another person, or when logging in on a device where you can't (or don't want to) install your password manager on (e.g. a PS4 or an Apple TV). In those cases, "puncture-foible-irish-ducat-rejoice" is a lot easier to handle than "jh&6dQ#F]9.Z>u^t]6u+".
The "symbol" password has more entropy for sure, but the actual security benefit is essentially non-existent. No one's going to guess either password, and I'm never using the same password in two different places anyway. The extra convenience is totally worth it.
EDIT: as other people have pointed out in the thread, another example would be badly behaving sites that prevent "paste" or use other techniques to block password managers. Much easier to type in those words then.
... for the same length. But length doesn't really matter unless you're manually typing it in, in which case many people will be faster at typing words than random characters.
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.
