So, a few characters in One Piece have suggested that devil's fruit users, along with all being weak to water, also heal up faster as well as being generally more durable. That is unrelated to power. Your fruit could be something weird (The ago ago no mi isn't a FRUIT, it's an EXPIERENCE), senseless ("I am a Butter Human, I produce butter than can stick to anything!" "Umm, I know you Charlottes were going for a theme but that is clearly not butter"), or some New World Devil Fruit Bullshit (See: Trafalgar " I can teleport... Just like a doctor" Law) but regardless it has restorative properties as well as simply making the user more resistant to damage.
This being the case, that would have meant that despite what he claims, Brook's fruit was not "useless" when he was alive. In fact, it's likely that his body was more resistant to the poison that killed him and his rest of his crew. So, see, Brook, your devil's fruit did do something prior to your death. It helped keep you alive long enough to watch each and every one of your nakama die in agony.
.... Oh, shit Brook I'm so sorry.
