Urine is stored in the bladder.
Holding on to urine causes a stretch on your bladder. When this occurs for a long time, the muscles of the bladder weaken and you become at risk of suffering Urine Incontinence, a condition in which you will have little control over urination. You could wet yourself unknowingly!
When you hold on to urine, your bladder becomes a breeding ground for bacteria. This is one of the causes of Urinary Tract Infections, a common cause of painful, frequent urination plus lower abdominal pains.
When your bladder is overfull, urine can retrograde or flow backwards into the ureters, potentially destroying them. It can also cause your Blood Urea Nitrogen to elevate, resulting in metabolic acidosis or electrolyte imbalance.
While it is advisable you to take in more water, it is also necessary that you pay attention to signals to empty the bladder to prevent kidney and bladder dysfunctions.