I was wondering if anyone else had wondered why Dragons have thumb claws on the ends of their wings?
I have asked this once before and never got an explanation that fit right.
I mean even admitting that everyone is probably just guessing, and no-one can say they really know and prove it, listening to some dip say that the thumb claw was possibly used to poke holes in crisp packets or force the tops of beer bottles didn't really convince me that that person had the answer.
I have had a couple of ideas, but I just wondered does anyone else have any thoughts?

Well, not all dragons do. I imagine that of those dragons that do have the claw on the joint between the halves of their wings, it's to protect the joint in battle or just day to day wanderings as if it breaks, flight will be impossible until it heals.

I used mine for climbing Obsidian.
I remember landing and putting my wings flat onto rocks to hold on.
When I do/did that I used the thumb claws to maximise my grip just like grasping a football with a human hand.
Wings and things
Lanntor

Since you say you have a couple of ideas and have asked this question before I am guessing that you would be one of those Dragon's Byakko mention who don't have them?
There are a few reasons for having the thumb claw, none of which are essential, but all of which are actually useful.
They help grip like Lanntor said. Rock faces are gripped and scaled much more easily and trees are easier to balance on too. They also help balance the wings while walking, stopping the Dragon from tipping forward.
In battle they make spiralling easier, helping the folded wings cut through the air, and they can be used to rip another's wing if the Dragon is a skilled flyer and is willing to risk reciprocal damage from the opponent, and, though not always best placed as a weapon, they are protective, they can make someone think again about getting close enough to damage a wing.
In ordinary flight they balance the wings, make swooping and manouvering easier and can help guard against accidental knocks and possible breaks.