What's the significance of having dehydration?

Depends on cause. Dehydration (a misnomer) can be caused by many conditions, including diseases, medications, and environmental factors. It is usually a loss of salt and water from the body. Mild to moderate depletion is usually easily corrected and may not require medical attention, while severe, recurrent, or persistent dehydration needs medical evaluation and treatment.
See below. Dehydration is due to loss of water and salt losses can also occur concurrantly] from diarrhoe, vomiting excessive sweating or use of medications called diuretics it can also occur if you do not drink adequate fluids to compensate for the loss you may develop low blood pressure, feel thirsty and weak.