If the app is not notarized by Apple, the user will get a warning message after opening .dmg file saying that "<APP_NAME> not opened: Apple could not verify <APP_NAME> is free of malware that may harm your Mac or compromise your privacy". And the user needs to close this popup and open a .dmg file again and then it's possible to install the app. It's a poor UX and most of the normal users will not install the app.

More info here: https://support.apple.com/en-ph/102445