The passwords unlock to different things. The login password on your Mac unlocks the hardware so that you can use it. The Apple ID password is used to access your Apple ID and all the related services.
I have a Mac with two separate user accounts on it, my personal account and my work account. They each have their own password. As it happens, both user accounts use the same Apple ID. This means both accounts share photos, contacts, iMessages and so forth. If I shared the computer with someone else, they could have their own user account with its own password and be logged into their own Apple ID.
Additionally, I can log into my Apple ID from pretty much any device that has a web browser. I log in to iCloud.com from my work computer (Windows) using my Apple ID and Apple ID password.
If your Mac never leaves your house and you don't have people you don't trust coming into your house, a weaker log in password for your Mac is less of a problem. But I still wouldn't recommend it. If the computer ever leaves your house, put a secure password on it, especially if it is a trusted device for two-factor authentication for your Apple ID.
Do NOT use an easy to guess password for your Apple ID.
I would worry less about "hackers" and more about people who might have physical access to your devices. Also, be aware of phishing scams.
Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
See also:
Effective defenses against malware and ot… - Apple Community
