I was just wondering why the resources page shows that PC users should download a special "Speedrun friendly" version of the game. What's the exact reasoning for this?

An older version of the game has certain bugs and quirks which have been patched out of the recent versions. As such, many runners opt to get the older version. You don't need it though, you can run the game on the current version.

Got it. Thanks!

Got it. Thanks!

The old version is also basically just for 51TE, 100%, and Max% currently.
The most recent patch version is needed for Any%, since it has introduced Ground Surfing, to allow gate skip.