What is the reason to use torx over Internal hex (Allen key)? Surely there must be a reason why it's used
When growing up it used to be that (internal) hex was for high strength but needed exact tool size, and Phillips/flat blade for low strength application with the benefit of any approximately sized tool will work.
Seems to me torx is in this awkward in-between where it's used for low-strength applications, but require exact tool size. Or it's used interchangeably with hex, requiring two sets of tools available at the work site (not fun if the work site is 30 meters up some ladders)
You can get more torque on a torx than an Allen. They're basically the same shape, but torx has longer contacts on the star points where the hex corners would be.
Torx is a lot less likely to strip out the tool or the fastener than Allen, especially at smaller sizes. Torx comes in a smaller variety of sizes than Allen does, so it's more difficult to choose a slightly wrong size. Especially with Allen being offered in both metric and moon landing fractionals, there's like, M0.8, and 14/93", one of which fits slightly loosely in the other and will sorta work, until you turn real hard on a stubborn bolt and round out the screw and/or driver.
In other words, it is my experience that you'll have a large pile of allens that you can't realistically tell apart, and a set of like 8 Torx tips from which you only use like three.
But not #1, or most of #2, in common stainless alloys. Even dry film lubed and drill tip is insufficient to prevent the torque from exceeding the (relatively low) yield strength, leading to stripped screws from standard driving in any wood denser than about G=0.45. There's simply not enough contact area to prevent it.
You really don't want to use stainless tools as they will plastically deform. Roberson is just not that great for torque-transfer geometry; it's advantages lie elsewhere.
You really don't want to use stainless tools as they will plastically deform
They are made specifically for stainless fasteners.
You'll frequently find them in the toolbags of aerospace professionals and industrial electricians and they are really the only folks that deal with a lot of stainless fasteners.
Bicycle and motorbike folks have lots of stainless hardware, too. And lots of time you'll see folks blame JIS/Phillips on ruining their fasteners, but sometimes it's just because they're using toolsteel tools on stainless fasteners.