Suppose that I am playing a character with proficiency with weapons. My hand consists of a bow (or crossbow) and two daggers. A character at another location encounters a monster and decides to make a combat check against it.
I want to help. But I can't use the daggers, since:
When playing another weapon, you may discard this card to add 1d4 to the combat check.
But I can discard my bow to give a bonus to their check.
Now that I've used the bow on the check, I can use the dagger as well?
And then the other dagger?
But only if I shot the monster with a bow/crossbow first?
Once you play the bow to add to a combat check at another location, you have played a weapon on the combat check, so you can then play a dagger. And then all the daggers you have in your hand if you want.
Dagger says you can use it to add to the combat check as long as you played another weapon on the check. It doesn't say it has to be your combat check. So once you've played a weapon (and you play a weapon with the bows when you add to a combat check at another location) you can play the dagger. And the daggers break the limit of 1 weapon on a check, and they don't set a limit for themselves.