When distributing software, it’s very useful to know which OS and which architecture the user is on in order to present them with the appropriate file.
Or you could let the user decide since you have no idea what system they want to execute the software on, just what system they are using to browse your site.
The overwhelming majority of the time (typically far higher than 99% of the time), if they hit a download button, they want it for their current platform.
And detecting their platform doesn’t stop you from presenting supplemental links for all platforms. In fact I’d say that’s completely normal.