Castration (neutering) is performed in male dogs to:

• Reduce overpopulation and unwanted dogs
• Decrease male aggressiveness and roaming behavior
• Decrease the incidence of undesirable urination behaviors
• Reduce diseases of the prostate and possible tumors of the perianal area
• Help prevent the occurrence of perineal hernias in the older male (or recurrence of a hernia following surgical hernia repair)
• Prevent, eliminate, or remove tumors involving the testes or scrotum
• Repair traumatic wounds when surgery may not be able to preserve the scrotum or testes
• Repair hernias involving the scrotum
• Alleviate or prevent obstruction of the urethra from bladder stones (as a component of prescrotal urethrostomy surgery)
• Control certain hormonal (endocrine) abnormalities
• Prevent propagation of puppies that might inherit certain defects or diseases

Castration of male dogs

Hypothyroidism in Dogs
Heartworm Disease in Dogs
Urinary Incontinence in Dogs

Lipomas in Dogs
Diabetes Mellitus in Dogs
Benign Skin Tumors in Dogs
Atopic Dermatitis in Dogs
Otitis Externa
Subcutaneous Fluid Administration
Giardiasis

A handy list you might want to refer to if you think your pet has gotten into something it shouldn't have:

Toxic Plant List