Why do molecules have to first transform to gas before doing a reaction?
Not all reactions require vaporization. This process looks like the Born-Haber cycle for cesium iodide, an exercise in Hess's Law usually used to fill in one of the missing values in the cycle that might be hard to measure (usually lattice energy, but not always). The Born-Haber cycle can also be used to drive general chemistry students crazy.

The $E_N$ "energy of neutralization" looks like the lattice energy, which is defined as the enthalpy/energy change accompanying the formation of a solid crystal lattice from the gas phase ions. In your case, the lattice energy of $\ce{CsI}$ is defined as the energy change accompanying the following reaction:

$$\ce{Cs+ (g) + I^- (g) -&gt; CsI(s)}$$

Because the lattice energy is defined for gas phase ions, you need the vaporization enthalpies for this process. Not all reactions require this vaporization step.