When is it necessary to include solvent effects?
Well, first of all, explicit is more expensive than implicit. So, most people do not go for the former if they can help it. So, in my experience, you want to use explicit solvent only when you have to, like in the formation of $\ce{H2SO4}$ from $\ce{SO3}$ and $\ce{H2O}$. Here, extra water molecules act as catalyzers [1]. As for the case keto-enol tautomerization, I guess the results are in good agreement with the experiments using implicit solvent.


The bifunctional catalytic role of water clusters in the formation of acid rain. Chem. Commun. 2017, 53, 3516-3519.