The app https://play.google.com/store/apps/details?id=com.kii.safe is free, so no. Some users might not want google to know they're using a particular app (this one a is a security one, so probably a higher proportion of users would care). There are over 10 million users from the play store, so 100,000 is not that high for sideloaders.

Plus, google might block downloads in some countries.

There are a number of sites that offer apk downloads for free apps: see http://apkpure.com/, https://apps.evozi.com/apk-downloader/, or http://apk-dl.com/. See also http://www.makeuseof.com/tag/download-apk-google-play-bypass... for more reasons people might not want to use Google Play.

Some devs sell APKs directly for users with devices without Play store. Samsung has their own app store. As does Amazon. And there is f-droid for open-source stuff. And I bet there are quite a few ones with regional importance.

Dealing with the play store can be a bit of a pain. The UI really isn't that great and the downloads are sometimes very slow. So it's honestly just a whole lot easier to side load sometimes.