Many people doubt that this would happen, because Russia looks weak right now, but imagine if there was stalemate in Ukraine and Russia got some time to recover.
After that it would attack one of the Baltic states and successfully capture its capital, and before NATO will even respond, Russia will say “that’s all what we wanted we don’t plan to go further”. Now (mostly Western European countries) might not want to get into a direct war with Russia and might let it slide. If that would happen it would basically erode NATO, because many of its members would start question what benefit they have for being members.
What sucks is that even if NATO would plan and would respond as it supposed to, if Russia would believe that this is the likely outcome they might still attack. So NATO needs to act and make clear that it won’t get divided in such situation.