For example, I only buy/use old high-end (for their day) business class laptops and put linux on them. They work for everything I need to write and surf.
I also bought an iPhone that was already a few years old and plan to keep it until it stops getting security updates from Apple. I also moved from Android to Apple not because I like iPhones better (I don’t, really), but because the years of security updates (versus maybe one or two, on Android) matters to me.
Benefit comes after the initial purchase whether you got it pre-built or made it yourself. Since then you can upgrade parts as needed after instead of doing one large system upgrade. So just buying a GPU if that’s what you need to upgrade instead of buying a whole new system. Or just dropping in a new CPU if your motherboard supports multiple generations like the AMD boards.
It’s one of those big impacts you see going with a replaceable desktop pc over a laptop. Especially if gaming, since if you find you want a a faster cpu or GPU now your only option is just buying a whole new brand new one paying for all the other parts again.