Originally Posted by Merlex
@grysqrl Doesn't proficiency allow you to add the Dex bonus to damage?
No. Proficiency with a weapon allows you to add your proficiency bonus to the attack roll. It has no impact at all on the damage.

Most (all, unless I'm forgetting something) weapon damage is one or more dice plus the appropriate ability modifier (str or dex, depending on the weapon). Magic weapons might add more. Offhand attacks don't get to add the str/dex modifier unless you have the dual wielder feat.