Tamsulosin Hydrochloride 0.4 mg tablets are primarily used to treat benign prostatic hyperplasia (BPH), a condition in which the prostate gland enlarges, causing difficulty in urination. It works by relaxing the muscles in the prostate and bladder neck, making it easier to pass urine and relieving symptoms such as weak urine flow, frequent urination, and the sensation of incomplete bladder emptying.