An importance of solubility is that it

Solubility is defined as the maximum amount of a solute that can dissolve in a given quantity of solvent at a particular temperature to form a saturated solution. Its importance lies directly in the fact that it determines the amount of solute that can dissolve in a given amount of solvent.

This knowledge is practically essential in:

• Preparing solutions of known concentration in the laboratory
• Predicting whether a precipitate will form when solutions are mixed
• Understanding purification techniques such as recrystallisation
• Pharmaceutical formulation (ensuring a drug dissolves sufficiently for absorption)

The other options describe colligative properties - effects that arise after a solute has been dissolved:

• Lowering the vapour pressure and raising the boiling point are consequences of having dissolved particles in solution, not descriptions of what solubility itself tells us.
• Knowing chemical reactions of a gas with a liquid relates to chemical reactivity, not solubility as a concept.

Solubility is fundamentally about how much dissolves, and that is its primary importance.