No, that's not necessary. You can download them straight to your phone. The 'side' is clearly in reference to the fact that you bypass the app store.

Yes, you can download APKs but I don’t remember that being common in the early days of Android.

This was before it had a download manager or file manager and using your computer to manage your phone was the norm.

The term predates Android anyways, goes back to the 90s I think.