This is similar to giving a hammer and needing to state to one is not responsible if the taker uses it to hit their head with.
I believe this is the wrong analogy to use here. It would be more like giving somebody a hammer and saying you are not responsible if the head flies off and injures you.
There is a concept in "things" you obtain called "fitness for purpose", so if you download open-source software like a registry cleaner, and it deletes your entire registry, the "WTFPL" doesn't absolve the authors of liability. Explicitly denying any kind of "fitness for purpose" or warranty helps shield the author from legal claims (even if they are meritless) because it kills it out of the gate. It's possible that the WTFPL license granter/author could still be sued and have to defend themselves in court because it isn't explicitly stated that there is no warranty or fitness for purpose.
Open source does not mean that it's "use at your own risk", which is why there are so many types of licenses and disclaimers.
Thank you for this very clear answer. Does the fact that someone willingly retrieves free code with no control of the provider changes anything? (vs a software vendor which provides code suitable for A and B, in exchange for money) I think closer to your hammer analogy where I would sue the producer of hammer XXX, a hammer I found on the street or which was given to me without involvement of vendor XXX.
No, this doesn't change anything. The software vendor here is the person who writes the code, even if they don't charge for it. They are the ones who license the software and they are the ones who take some level of responsibility for it. In this case, the software author is the "hammer producer" that you would sue...
If you buy "Registry Cleaner XL" from Best Buy and it bricks your computer, you don't sue Best Buy, you sue the person who authored the software. In pretty much every case, the author is the responsible party, not the vendor, and it is the author who licenses it, not the vendor (the vendor just acts as an intermediary).