Those keys are mostly 'volume keys' which are meant for devs. Some people sign up as devs and pay some joining fees and in return they get lots of Office keys and sell it on their website. You can't use those product keys if you want to use Office on a different device as it'll not bind to your MS account or can't be activated on a different machine. You need to buy another key to use it on a different machine.
I bought JBL earbuds and didn't register my product on their website as most companies don't require it so I basically ignored that. After 7-8 months of usage one bud was discharging quicker than the other one so I tried to claim warranty but they refused, they said that my product isn't registered in their system. They told me to register the product first then apply for a warranty claim.
Yeah, if a company forces me to do so (Razer, JBL). Most companies don't require registration for warranty, at least in my country. All you need is the original invoice. When you want to claim a warranty, you need to fill out a form on that company's website which asks for your details like name, phone number, email, and physical address, alongside the original invoice. That's it, no prior registration required.
That's an example I gave. I use Solid Explorer.