How much damage could a single battleship (or a task force of one battleship and one heavy cruiser) do to convoys of 30+ ships? Or was the value more about proving that the threat existed and requiring additional resources for convoy protection?
There's quite a bit on this in our FAQ, which I'll link below, but the short version is: yes, the Bismarck and Prinz Eugen would have been quite deadly to Atlantic convoys, which were escorted with fairly light ships and never enough of them. The British navy had very good reasons to commit considerable resources to sinking the two ships -- this thread in particular is a great write-up on this.
FAQ link:
https://www.reddit.com/r/AskHistorians/wiki/faq/militaryhistory/wwii/uk#wiki_royal_navy