well, speed would be the main reason, and you know. if you needed to install Windows 10 on another computer, it would be boring to download it again and again and again.
also you can include 32 and 64 bits so you will be able to keep it for other computers, just in case, you know.
and about the reset option, is nice. but sometimes it fails. also I like to boot on a Linux distro and move everything, program files, windows, appdata, etc, into an empty folder. so I have more control on where my files are and what I want. so it depends if you trust the reset option or you want to boot from usb and install it yourself.
There is no problem about using both methods, for some people just reset would be quicker and better, I just like to have more control because I want to make sure I don't lose any file in the process. but Windows 10 on a usb install like on 15-25 minutes. I remember the reset option took a little longer than that. so either way it's okay, using reset option would work 99.9% of the time as expected.