A much easier way to get to the Upside-Down.
Sure, you could build a secret research laboratory with a big creepy portal and climb through a bunch of slimy, scary branches into a secret Upside-Down world.
Or, you could rescue a girl with psychokinetic abilities from said secret research laboratory and then break into your school and build a sensory-deprivation tank in the gym and immerse her in it so she can get to the Upside-Down world.
OR you could just strap yourself into one of these things and flip yourself upside down. So maybe it's not as exciting as rescuing a girl from a secret lab, but things will still look pretty weird once you're flipped over.
Plus, it feels pretty good on your back! And you can't say that about climbing through a bunch of slimy tree branches.