@GNU2Linux :- Hallo.....and 
to the "kennels". Nice to have you here with us.
Um.....as mentioned above, it IS purely down to personal choice. As @trawglodyte says, he's been putting Puppies onto his internal drive for the simple reason that, up until now, he was always used to performing full installs to his main drive with all the other distros he's tried out. And yes; for some folks, the whole concept of running an entire OS from a USB stick takes some doing to "get their head around it".
Many people have difficulty adjusting to this rather different way of doing things. Some, however, take to it like a duck to water.....
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@mikeslr mentioned the best way of approaching the Puppy-to-hard-drive paradigm up above. Unlike other distros, which invariably require their own, dedicated partition for them and them alone, multiple Puppies can quite happily "share" a single partition between them.
If you wanted to, say, run 3 different Puppies from the same partition then you would create three separate directories (Linux-speak for a Windows 'folder'), and give each one a unique, distinctive identifying name (usually the name of the distro itself is quite sufficient). You retain the same "frugal"-type installation method as you would for a USB stick; if you like, think of each sub-directory as an individual USB stick, if that makes it simpler to visualize.
Unlike most distros, which are constantly reading-from/writing-to their partition, and only loading stuff from the hard drive as & when it's needed, with the Puppy frugal install method you basically load the entire OS into RAM.....and then run it from there. An area of RAM is 'set-aside' for caching additions, configuration changes, etc.....then you have three options:-
- Save these changes back to the 'save'-folder at regular, specified intervals
- Save back to the 'save'-folder continuously, as & when they're made
- Set things up so that Puppy ONLY 'saves' this stuff to the 'save'-folder as & when you TELL it to
All of the above are recognized by Puppy as different operational 'modes'. If you stay with us for any length of time, you'll see these mentioned quite regularly, as different people figure-out the best way for them, personally, to use Puppy. What works for ME will not necessarily work for YOU (and vice-versa).
Until very recently, computer RAM was by far & away the fastest-performing component of any system. However, the current crop of modern, nVME NAND-flash drives (think supercharged SSDs!) are now reaching the point where they're starting to overtake RAM itself in terms of sheer speed. All of which opens up a whole new raft of possibilities.... 
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IF you want 'portability' then it goes without saying that the 'save'-folder will need to be on the USB stick along with the rest of the OS. If, however, you will always run this USB Puppy from the same machine, it makes more sense to create the 'save'-folder to run on your internal HDD/SSD/nVME/whatever, because these media are far faster than most USB sticks.
With any Puppy employing the 'frugal' method of operation, all you're really doing is use that stick/directory/whatever to 'store' the compressed files that make up the OS until such time as you boot it.....at which point it's all decompressed into a virtual file-system that's created, 'on-the-fly', in RAM as & when it's needed. I know it all sounds very complicated, but once you begin to understand the unique, 'Puppy' way of doing things it will actually start to make sense..! 
At the end of the day, as far as the average user is concerned, a 'Puppy' will work exactly the same in operation as any other OS. You don't HAVE to understand the clever, technical tricks that make it all possible under the hood.......not, that is, unless you're curious and WANT to.
Mike. 
