There are a lot of locked doors in witcher 3, do they serve any purpose at all?

Is there a skeleton key or something that will provide access?

There is no skeleton key.
There are 42 quest keys you can find along the way.
Although not all are door keys
Some doors have no key & require you to find an alternate way in.
Some locked doors will be unlocked without a key, just by quest progression.
Some just can't be entered.