If the user agent reports 32 bit, when you go to download an installer, the page can present you with the 32 bit version. Likewise with 64 bit (from the early days when 32 bit was more common). Same goes with reporting that it's x86 vs ARM.

So now if you are on an old 32 bit machine and try to install new software from the web, it'll give you a 64 bit installer that you almost certainly won't even be able to run on your machine.

Now with linux that's less of an issue given that installs on linux already depend on a ton of stuff (and linux users tend to understand the difference between 32bit and 64bit)