I use a pretty simple password in my PC and Laptop, since, in theory, I'm the only one with access to them (oh, and my wife, of course).

I just don't have ssh enabled and the root passwords are ridiculously strong.

Physical security is more important than software security on laptops. In a public space it's enough to have a shit password as long as there is one.

For ssh ports or remote desktop connections that don't use pairing you definitely need a strong password. For local WIFI connections it's not as important as your WIFI security though.

It doesn't need to be crazy secure as there is a retry delay enforced by the OS.

If you are talking about LUKS2 then go with a passphrase

For context, my threat model doesn’t need to account for real people breaking in and accessing my computer, the damage would be very contained.

I mean if you don't have open ssh ports on your computer or whatever I don't think you need a strong password, given that you're not concerned about physical access. I would say that at the very least have a reasonably secure root password (/user password if you're a sudoer/anyone else who can get root permissions with your user account) because if you end up with some malware on your computer that can, say, enter passwords, you don't want it to be ridiculously easy to bruteforce.

Not at all. If you think about security like if a thief was to steal your data then the only thing protecting you would be disk encryption. A password for login won’t prevent someone from taking your disk.

Just like a chain, strength is determined by the weakest link

Use secure passphrase for ALL your devices on your network

To address the "why":

A user account, as defined by a username/password combination, can be used to access resources on the machine without logging in interactively on that machine. In a perfect world, you would only ever log in interactively on the machine using an account with restricted permissions, and when you needed to do "administraty" things, you would provide separate admin credentials at that time (sudo, runas, whatever your OS of choice supports).

Bonus question - what are the risks of having a weak password on a root user on a spare laptop on the same network as my main device that is used exclusively for web browsing?

If someone is able to compromise that root user on Machine A, then they may be able to leverage Machine A as a platform to attack any other devices on your network, or make Machine A into a zombie in their bot army to attack other targets anywhere, send spam, whatever malicious shenanigans they desire. (I know that's pretty simplistic, there's a whole lot of details left out, but that's the gist of it.)

Also, nobody has yet mentioned the PIN option. I know that Windows machines (at least some of them, depending on configuration) allow you to configure a PIN for logon for local accounts. This PIN is only stored locally on the machine, and is not transmitted anywhere else. It's basically a "shortcut" to the full password, and I think it can only be used for interactive logon.

Home computer? I wouldn't even have a password. If someone has physical access to your PC there are ways for them to get what they want no matter how strong your password is. You just need it to be secure enough to prevent specific people you know have access from getting in. Family, friends, whatever. So basically the only risk is that you use a password that they know you use because you shared an account with them or something.

This is an interesting table for password cracking based on password hash..

Using random characters is dumb.

It incetivices you to write it down (which in and of itself isn't a completely terrible idea).

As an IT guy, let me tell you, STOP using random passwords!

Use passphrases instead, it is far easier to remember a sentence than a random string of letters and numbers.

Not a direct answer to your question, but you can use passphrases instead to make it easier and faster to type. This is assuming your only option to log on to your computer and unlock your screen is to type in your password (e.g. no biometrics or hardware token options).