Long story short, to install an OS [i.e. Windows] you usually need a bootable USB stick, or an optical drive with a bootable disc. It is possible to setup Windows without running Windows setup -- you basically just copy everything needed to a hard disk / SSD, and it *should* boot -- but to copy those files you either need a bootable USB stick / drive, or you need to connect the target drive to another PC / laptop. If neither of those is an option, you can try booting to the network adapter & install Windows using the setup files over a network, but that is going to get a bit involved. And that's where WinToHDD Pro comes in... *in some cases* it lets you get away without using anything USB, &/or it may make it easier to copy the files Windows needs to an empty partition without having to bother using DISM on the command line. But it's not something most people, or *I think* many IT dept.s would use.
Note: getting Windows to boot after swapping disks, or if you copy the files rather than running setup, has become something of a can of worms. It simply may or may not work. You can try a repair utility, like those on the Macrium Reflect USB stick, and you can try Windows BCDBoot to create new boot files, but neither is anywhere close to a guaranteed fix. The only thing that is is booting to & running Windows setup and installing Windows fresh.
Some Background
If you have a PC/laptop with no OS [Operating System] installed on an internal drive, you need to install an OS [usually Windows or Linux] *fresh*. This will install everything the OS needs, and anything on the destination partition(s) will be overwritten. You can also do a fresh OS install when/if you just want to start over, getting rid of everything associated with your current OS. And you'd do a fresh install if you were adding another OS to your PC/laptop, so for example you could boot into Win10 or 11, or if you were setting up a new VM [Virtual Machine]. Now usually you'd have the OS setup files on a bootable USB stick. If the internal drives are blank, there's nothing to boot to to start the setup routine. If you're adding say a 2nd copy of Windows, you can't run setup in Windows, since that would upgrade or reinstall the copy of Windows you're running -- not install a 2nd copy. If you want to replace everything with your current copy of Windows, running Windows setup in Windows to do a reinstall will give you the choice whether to keep your existing files etc., but things like drivers will often be reused, which can be bad if you're starting over because of malware. Instead of running setup you can reset Windows, and this can download everything necessary, but again you might be reusing things like drivers, and you won't be wiping the partition before before Windows is copied to it.