FDroid offers the guarantee that the apk was built from source, and at least some marginal safety in terms of having passed their security and analytics/ads checkers.

You lose all that if you go directly to source.

Obtainium makes it easy to download apps directly from their webpages. Many of them have ready configurations you can simply add: https://apps.obtainium.imranr.dev/

With F-Droid (at least the main repo) you get extra guarantees that the apk is built by them. A thirdparty store also updates your apps, so you're not stuck at an old outdated version. I do use both F-Droid and Obtainium though.