On propositional formula satisfiablitiy
When converting an arbitrary formula into a CNF, you are introducing extension variables. These are new variables that represent subformulas of your formula.

Let us take the following simple formula as an example:

$$ A \lor (B \land C). $$

In order to convert this into a CNF, we need to replace $B \land C$ with an extension variable $\alpha$ which has the same truth value of $B \land C$. We can easily express "$\alpha = B \land C$" as a CNF (or as a DNF). After replacing $B \land C$ with $\alpha$, we obtain the formula
$$
 (A \lor \alpha) \land \text{"$\alpha = B \land C$"}.
$$
(You need to replace "$\alpha = B \land C$" with a CNF.) This formula is equisatisfiable with the original formula: the original formula is satisfiable iff the new formula is satisfiable. Therefore if we are interested in knowing whether the original formula is satisfiable, we can instead ask whether the new one is.

In the transformation outlined above, we used the fact that the class of CNFs is closed under conjunction. That is, we could express add the condition "$\alpha = B \land C$" to our formula while retaining its CNF form. The same is not the case for DNFs, which are closed under disjunction but not under conjunction.

Indeed, consider the opposite situation, in which we want to convert $A \land (B \lor C)$ into an equisatisfiable DNF. We can easily express $\alpha = B \lor C$ as a DNF; the problem is that $(A \land \alpha) \land \text{"$\alpha = B \lor C$"}$ is not a DNF, since we joined $A \land \alpha$ and $\alpha = B \lor C$ with the wrong connective.