When is it necessary to include solvent effects? When modelling a process like keto-enol tautomerization or SN2 ring opening reaction, I am unsure as to what factors need to be considered when deciding whether solvent effects need to be accounted for. Because if they are important, a solvation approach, implicit or explicit is chosen to model the process. 
For example, in a keto-enol tautomerization, solvent effects are important as the equilibrium is maintained by the solvent. 

What I was thinking is that using the explicit water model, as few as  1 to 2 water molecules can be used to model the tautomerization. But many studies are showing that continuum solvent models (implicit) are the benchmark for keto-enol tautomerizations. I don't understand why and what factors determine this?
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.