It’s one less thing to do. Linux users are much more inclined to tinker than the average PC user; the nature of the OS encourages (and in some cases mandates) such things. But, sometimes, letting someone else do the basic heavy lifting can be worth paying for when you’re in a hurry or have other things on your mind.
A different range of options. Those who build their own PCs and preload it with one kind of Linux or another are typically building a desktop or home-theater style system or a “mini-PC” (such as a Raspberry Pi). Rarely do they build a laptop, simply because of the relatively few options available for the latter, apart from buying an OEM or name brand system wholesale. If you want a laptop, it’s a lot easier to buy from a Linux vendor.