http://g4c3eya4clenolymqbpgwz3q3tawoxw56yhzk4vugqrl6dtu3ejvhjid.onion/recommendations.html
A: Because it allows you to keep using almost all apps that you may want to or need to use.
A lot of apps depend on Google Play Services and may straight up refuse to work without it, like some banking apps for example.
microG fills this gap by acting as a drop-in replacement for Google Play Services and because microG is unable to work properly on plain LineageOS,
you probably want to use LineageOS for microG instead.