So what is the point of the weapons critting on "objects"? so it takes less hits? oh boy what a bonus.
Some chests, doors and walls will have toughness, which means they don't take damage from any attacks that deal less than a certain amount thus ensuring critical hits can give you enough damage to overcome these thresholds.
Sometimes you also could use the speed of destroying something with crits. Like for example you walked into a trap and your party's perception check suceeded too late, but you act fast enough to attack and destroy the trap mechanism before you get harmed. Or say you're in combat and you target the enemies weapon or armour to destroy it rather than harm them (If your DM allows this).
Also, in prior editions (Or maybe just with certain DM's) Constructs can be considered "Objects" - With the reasoning that they're not actually alive, they're just a bunch of mechanical parts moving together, thus would still be considered an object rather than an entity.
Essentially, in BG3 the adamantine weapons are a vestigial bonus. They make sense in TT D&D where their property is actually useful (Also, you're not always so likely to find as many powerful magic items in your average campaign). In BG3 itself its mostly pointless besides smashing a few chests/doors more easily.
